Fix #5368 - Improve test efficiancy

There were a number of tests where lots of models where being iterated through
and then at the end committing them. The problem is all the changes had to be
kept in memory or on disk somewhere before they were commit to the database. This
fix commits each change as it's being made, allowing resources to be freed and fixing
some of the inefficiencies

mediagoblin/db/migrations.py

@@ -1371,7 +1371,8 @@ def migrate_data_foreign_keys(db):
             temp_object=target_gmr.inserted_primary_key[0]
         ))
 
-    # Commit to the database.
+        # Commit to the database. We're doing it here rather than outside the
+        # loop because if the server has a lot of data this can cause problems.
         db.commit()
 
 @RegisterMigration(30, MIGRATIONS)