test: fix assert when running test-cgroup in container

test: fix assert when running test-cgroup in container

When running in nspawn and thus from the namespaced root cgroup:

Paths for test:
//test-a
//test-b
src/test/test-cgroup.c:77: Assertion failed: Expected "path == test_b", got "/test-b != //test-b"

Use path_simplify() in the test like the code does, so that the results
always match