Removed available_apps on TestCase subclasses.

TestCase subclasses are wrapped in a transaction that prevents any data from
being persisted between test runs andi thus don't require limiting the tables
to be flushed to a subset of available apps like TransactionTestCase subclasses
do.

tests/fixtures/tests.py

@@ -689,7 +689,6 @@ class NonExistentFixtureTests(TestCase):
     """
     Custom class to limit fixture dirs.
     """
-    available_apps = ['django.contrib.auth', 'django.contrib.contenttypes']
 
     def test_loaddata_not_existent_fixture_file(self):
         stdout_output = six.StringIO()