tests: Remove the SWHTag mock, use dulwich.objects.Tag instead.
This mock was clunky because it didn't actually behave much like dulwich's Tag.
Additionally, a future commit will need to access the as_raw_chunks() method of ShaFile objects, so SWHTag isn't suitable anymore as it would need to diverge even more by implementing its own serialization.
Migrated from D6865 (view on Phabricator)