Currently, we do not clean up after ourselves after tests in core::rmdir
have created new files in the directory hierarchy. This may leave stale
files and/or directories after having run tests, confusing subsequent
tests that expect a pristine test environment. Most importantly, it may
cause the test initialization to fail which expects being able to
re-create the testing hierarchy before each test in case where another
test hasn't cleaned up after itself.

Fix the issue by adding a cleanup function that removes the temporary
testing hierarchy after each test if it still exists.