test-path: do not fail the test if we fail to start a service because of cgroup setup

test-path: do not fail the test if we fail to start a service because of cgroup setup

The test was failing because it couldn't start the service:

path-modified.service: state = failed; result = exit-code
path-modified.path: state = waiting; result = success
path-modified.service: state = failed; result = exit-code
path-modified.path: state = waiting; result = success
path-modified.service: state = failed; result = exit-code
path-modified.path: state = waiting; result = success
path-modified.service: state = failed; result = exit-code
path-modified.path: state = waiting; result = success
path-modified.service: state = failed; result = exit-code
path-modified.path: state = waiting; result = success
path-modified.service: state = failed; result = exit-code
Failed to connect to system bus: No such file or directory
-.slice: Failed to enable/disable controllers on cgroup /system.slice/kojid.service, ignoring: Permission denied
path-modified.service: Failed to create cgroup /system.slice/kojid.service/path-modified.service: Permission denied
path-modified.service: Failed to attach to cgroup /system.slice/kojid.service/path-modified.service: No such file or directory
path-modified.service: Failed at step CGROUP spawning /bin/true: No such file or directory
path-modified.service: Main process exited, code=exited, status=219/CGROUP
path-modified.service: Failed with result 'exit-code'.
Test timeout when testing path-modified.path

In fact any of the services that we try to start may fail, especially
considering that we're doing some rogue cgroup operations. See
https://github.com/systemd/systemd/pull/16603#issuecomment-679133641.