Fixes #17777 and makes tests run again.

Adds a salted MD5 hasher for backwards compatibility.
Thanks gunnar@g10f.de for the report.

Also fixes a bug preventing the hasher tests from being run during
contrib tests.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@17604 bcc190cf-cafb-0310-a4f2-bffc1f526a37