test_describe_tests: remove 'manual test stubs'

test_describe_tests: remove 'manual test stubs'

Use a combined strategy: for test classes, use setup
to manage the list of patchers, also starting them
in setup because all the class-based tests use the same
fake fs data.

For the file-scope test functions, use @patch to manage them,
because the filesystem data changes with each test.

Signed-off-by: Dan Mick <dan.mick@redhat.com>