[1.2.X]Converted m2m_signals from doctests to unittests. Thanks to Gregor...

[1.2.X]Converted m2m_signals from doctests to unittests.  Thanks to Gregor Müllegger for the patch.  We have always been at war with doctests.  Backport of [14548].

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14549 bcc190cf-cafb-0310-a4f2-bffc1f526a37