This should catch the bug fixed by "worker: ignore meaningless exit status
indication returned by os.waitpid()."
Before, worker.py was untested since test repositories are relatively small.
Many tests didn't change back from subdirectories at the end of the tests ...
and they don't have to. The missing 'cd ..' could always be added when another
test case is added to the test file.
This change do that tests (99.5%) consistently end up in $TESTDIR where they
started, thus making it simpler to extend them or move them around.