[1.2.X] Fixed #15111 -- Ensured that the auth, contenttypes and sitemaps tests...

[1.2.X] Fixed #15111 -- Ensured that the auth, contenttypes and sitemaps tests will run when the sites app isn't installed. Thanks to Waldemar Kornewald for the report and draft patch.

Backport of r15418 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15419 bcc190cf-cafb-0310-a4f2-bffc1f526a37