Fixed #16412 -- Prevented a `contrib.auth` test from failing in the potential...

Fixed #16412 -- Prevented a `contrib.auth` test from failing in the potential case where `contrib.sites` was not installed. Thanks to haras for the report and to Aymeric Augustin for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16717 bcc190cf-cafb-0310-a4f2-bffc1f526a37