Fix nondeterministic testmvcc failures

This test wants to rely on OIDs that are assigned to various objects.
OIDs are assigned by Pickler.persistent_id(), which gets called during
graph traversal in a nonspecific order (courtesy randomized hashes in
Python 3), so if we want to be sure to have the right OIDs assigned to
the right objects, we need to call connection.add() manually or commit
after adding every object.