Polish testcases/urls.py
Polish testcases/urls.py

* Move the deprecation warning for URLsMixin into
  the __init__ method so it doesn't show up when the
  class is merely imported.
* Add two more URLsTestCase equivalent classes, one
  for each of the new mixins, with better names.
  This also means we can deprecate URLsTestCase
  by making it use URLsMixin.

incuna/incuna-test-utils,incuna/incuna-test-utils