Split out the DmarcDnsTestsMixin into a new testing module.
Review Request #8415 — Created Sept. 19, 2016 and submitted
The DmarcDnsTestsMixin is pretty useful outside of Djblets. In order to make this more available, this has been split out into a new testing.py module. Consumers can pull this in to simulate different DMARC conditions in their own unit tests.
Unit tests pass.
Tested this as well in Review Board.
