Fixed #8286 -- Simplified the m2m intermediate serialization test to make it...

Fixed #8286 -- Simplified the m2m intermediate serialization test to make it easier to debug, and less prone to failures caused by test execution order. Thanks to jarrow for the report, and Karen Tracey for her help confirming the problem.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8347 bcc190cf-cafb-0310-a4f2-bffc1f526a37