Use URLsTestCase for urls tests

https://github.com/incuna/incuna-test-utils/blob/0dc84650b2929d31c054882ad67570fda6f1ffb9/incuna_test_utils/testcases/urls.py#L24

https://github.com/incuna/django-user-management/blob/1ee5fb913deb911c967fa3844847ec1a85597fae/user_management/api/tests/test_urls.py#L5-L12