Add tests for untested code

* Test `BaseIntegrationTestCase.render_to_str` and `BaseIntegrationTestCase.access_view_and_render_response`.
* Refactor test_integration.py to take advantage of pytest fixtures and decouple the tested TestCases from the test methods.
* Improve coverage to 100%.
It caught an unnecessary db access in `test_render_to_str` - I didn't need a real `request.user`, an `AnonymousUser` was sufficient.