Set up custom coldfront test runner to set fake seed and stress level.

[Eric-Butcher]

This PR includes a custom test runner for coldfront which is a subclass of the default DiscoverRunner. This custom test runner adds flags on the command line to specify the seed used for the random value generation with faker as well as an argument to give a stress level to the tests. This custom test runner will also generate and report each time the test suite is ran what the seed used for faker was regardless of whether or user specified it or not. This closes issue #721.

The only concern with this is that even when specifying a seed manually, there might be an issue if tests are ever run in parallel that might cause inconsistencies.

Some things that might want to be modified:

1. What to name the file that stores the new test runner code
2. Whether float or int would be preferred for the seed
3. Whether to keep the constants for FAKE_SEED and STRESS_LEVEL as variables imported from the module or to make these settings using django.conf.settings

If you like it as is then it should be ok to merge in.

[ANekhai]

I just have one quick formatting question. The actual content of the test_runner file looks good to merge for me.