The DynamicURLResolver tests had an order dependency issue. They assumed
that no prior test would populate the URL resolution caches. This was a
terrible assumption, and the recent extension work broke this.

We now make sure to clear the URL caches before and after the test,
fixing this particular problem. Some of the inner workings of the test
have also been cleaned up, making the test more self-contained.

All unit tests pass on Django 1.6 and 1.8 through 1.10.