Refs #20392 -- Loads fixtures once within TestCase

Following suggestions on https://code.djangoproject.com/ticket/20392 and https://groups.google.com/forum/#!topic/django-developers/deADv5ZaL6o, this PR takes advantage of the class level transaction in `TestCase` to load fixtures within `setUpClass`.
Can we move this conditional out of the loop? (I see it was copied from `_fixture_setup`.