Refactor the use of archive Storage object for testing

Remove any use of swh.model.tests.swh_model_data.TEST_OBJECTS, and keep a single fixture for
ArchiveInterface objects based on ArchiveStorage (using swh_storage fixture as parameter).
This avoids duplicating every test for each ArchiveInterface implementation. All
implementations of ArchiveInterface are tested against each other in test_archive_interface.

Also, ArchiveStorage now has a storage attribute to access the underlying StorageInterface
object, so that test no longer needs to receive archive and swh_storage separately. Each
test is responsible for filling archive. storage with the required data specific to the test.