[1.2.X] Fixed #12893 -- Added tests to validate that the right queryset is...

[1.2.X] Fixed #12893 -- Added tests to validate that the right queryset is always used in model admins. Thanks to mk and Julien Phalip for their work on the patch.

Backport of r15578 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15579 bcc190cf-cafb-0310-a4f2-bffc1f526a37