[1.2.X] Fixed #14458 -- converted m2m_regress tests from doctests to...

[1.2.X] Fixed #14458 -- converted m2m_regress tests from doctests to unittests.  We have always been at war with doctests.  Thanks to Gabriel Hurley for the patch.  Backport of [14205].

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14206 bcc190cf-cafb-0310-a4f2-bffc1f526a37