tests: vcs: use _BackendTestMixin instead of duplicated BackendBaseTestCase in test_filenodes_unicode_path

test_inmemchangesets.py hosts a second base class in addition to
_BackendTestMixin, which is basically a duplication we want to get rid of.

As that duplicated base class BackendBaseTestCase is still in use by
test_filenodes_unicode_path.py, we first need to let that test use the main
base class.