Cleanup and test improvement

[jone]

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?

[lukasgraf]

:+1: (Now I see what you meant by "unused variables". Those _ = assignments I originally did to indicate that the convert() method returns a result that would actually be used in production, and is excplicitely ignored in those tests. But looking at it now it just adds clutter to the tests, so good call for removing it)