[1.2.X] Converted model_inheritance doctests to unittests. We have always...

[1.2.X] Converted model_inheritance doctests to unittests.  We have always been at war with doctests.  Backport of [14348].

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14349 bcc190cf-cafb-0310-a4f2-bffc1f526a37