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%.
Will this work consistently on both python 2 and 3?  I seem to recall we had problems with that in the past.