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's part of [pytest-django](http://pytest-django.readthedocs.org/en/latest/helpers.html#pytest-mark-django-db-request-database-access).