[1.3.X] Refs #15573, #15850: Added a check for whether the sites app is...

[1.3.X] Refs #15573, #15850: Added a check for whether the sites app is installed when creating the test database, in order to work around a bug in get_model.  Thanks to adsva and carljm.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.3.X@16062 bcc190cf-cafb-0310-a4f2-bffc1f526a37