Converted model_inheritance_select_related tests from doctests to unittests. ...

Converted model_inheritance_select_related tests from doctests to unittests.  We have always been at war with doctests.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14181 bcc190cf-cafb-0310-a4f2-bffc1f526a37