[1.2.X] Converted the model_regress doctests into unittests. We have always...

[1.2.X] Converted the model_regress doctests into unittests.  We have always been at war with doctests.  Backport of [14614].

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14617 bcc190cf-cafb-0310-a4f2-bffc1f526a37