Cleanup and test improvement

Contains:
- Some pyflakes (removed unecessary variables) and indentation
- Tests: be more specific on what raises an exception when using `self.assertRaises` context manager
- Remove unnecessary utils testing layer, which is actually just unit testing and no layer is needed
- Add tests for the ZCML directive
- Configure Tika in the testing layer and add an integration test, verifying that the conversion actually works
- Use integration testing only, we never use a browser, so there is no need for functional testing.

@lukasgraf can you take a look at these changes?