Split out the DmarcDnsTestsMixin into a new testing module.

Review Request #8415 — Created Sept. 19, 2016 and submitted — Latest diff uploaded

Information

Djblets
release-0.9.x
73b48a5...

Reviewers

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.