tests: add code to handle HTTP digests on the server side
It's not hooked up yet. Mostly this was cargoculted and simplified from some
python.org code[1]. It's not trying to test the security as much as it is
trying to make sure that clients are sending out the right data when challenged.
(And they aren't on py3.)
[1] http://svn.python.org/projects/sandbox/trunk/digestauth/digestauth.py
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