Converted m2m_signals from doctests to unittests. Thanks to Gregor Müllegger...

Converted m2m_signals from doctests to unittests.  Thanks to Gregor Müllegger for the patch.  We have always been at war with doctests.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14548 bcc190cf-cafb-0310-a4f2-bffc1f526a37