Models are now attached to any application they're defined in. Since
not_installed was inside app_loading, these models were mistakenly
attached to app_loading. The test that used them passed accidentally
when run after EggLoadingTest because that class' tearDown() method
replaces apps.all_models['app_loading'] by a copy of itself, while
it should remain the same as apps.app_configs['app_loading'].models.