Fixed #13389 -- Ensured that the app_loading test clears out the app cache at the end of each test, so that it doesn't interact badly with flush and other introspected database commands. Thanks to Karen for the report, and Ramiro Morales for the debugging hints.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13011 bcc190cf-cafb-0310-a4f2-bffc1f526a37