Attempt to fix flaky AMO tests by not relying on ExtraAppTestCase anymore

ExtraAppTestCase causes problems depending on import order, because
any models with FKs that is imported get cached into its related
model's _meta._related_objects_cache.
Merge pull request #1267 from diox/fix-flaky-tests

Attempt to fix flaky AMO tests by not relying on ExtraAppTestCase anymore