Improved test isolation of the admin tests and assigned custom admin sites to

prevent test order dependant failures.

This involves introducing usage of `TestCase.urls` and implementing proper
admin.py modules for some of the test apps.

Thanks Florian Apolloner for finding the issue and contributing the patch.

Refs #15294 (it solves these problems so the fix for that ticket we are going
to commit doesn't introduce obscure and hard to reproduce test failures when
running the Django test suite.)

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16856 bcc190cf-cafb-0310-a4f2-bffc1f526a37