Converted regressiontests/forms/fields.py to unittests

      This makes merging in the soc2009/model-validation simpler and rids us a
little bit more of awful doctests. Thanks Honza!

git-svn-id: http://code.djangoproject.com/svn/django/trunk@11858 bcc190cf-cafb-0310-a4f2-bffc1f526a37