Fixed #16593 -- Refactored proxy_model_inheritance fixture setup to minimize the chances of leaving a modified INSTALLED_APPS setting for tests ran after it if setUp fails. Thanks Jim Dalton for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16593 bcc190cf-cafb-0310-a4f2-bffc1f526a37