Improve test coverage

[HerrBertling]

Right now, the test coverage across the different packages is rather low:

I'd like to improve this quite a bit to make future work on the packages a bit more secure. Can't pin down a definite percentage I want to achieve since a) I don't know whether I'm able to test everything and b) I currently have no idea which parts need testing and which can do without 😆

Something at ~50% seems like a good first shot, though.

@svewag @escapedcat @Xiphe @fdietz If anyone of you has input here, feel free to comment :)