Fixed #13638 -- Refactored the serializers_regress tests to avoid the use of flush, and make better use of the transactional capabilities of django.test.TestCase. Thanks to Alex Gaynor for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13308 bcc190cf-cafb-0310-a4f2-bffc1f526a37