test-path: start infinite sleep instead of a short command

test-path: start infinite sleep instead of a short command

The test sometimes fails, e.g. in bionic-s390x ci. I think it might be because
the service binary exits before we get a chance to notice that it is running:
13:59:31 --- Listing only the last 100 lines from a long log. ---
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 4639845)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 4539608)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 4439376)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 4338946)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 4238702)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 4138424)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 4038116)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 3937835)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 3837553)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 3737250)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 3636934)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 3536622)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 3436318)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 3336021)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 3235730)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 3135468)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 3035158)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 2934855)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 2834541)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 2732511)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 2632255)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 2532014)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 2431746)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 2331438)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 2231213)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 2130952)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 2030663)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 1930428)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 1830172)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 1729906)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 1629652)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 1529368)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 1429110)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 1328852)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 1228593)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 1128320)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 1028083)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 927824)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 827564)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 724935)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 624664)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 524411)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 424124)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 323853)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 223585)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 120356)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: 18053)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 line 293: path-unit.path: state = running; result = success (left: -82385)
13:59:31 line 293: path-mycustomunit.service: state = exited; result = success
13:59:31 Test timeout when testing path-unit.path

It seems test/test-path/path-service.service wasn't actually used for anything.