Structure tests like your modules & add plotting tests & fixtures

[Zethson]

This is a bit more work and not super urgent but I'm writing it nevertheless:

1. Please consider structuring your tests so that they're in modules (folders) that correspond to the structure of the actual liana-py package. This helps find tests and navigate things. Commonly done.
2. Please consider increasing the test coverage by also covering plots. At least I didn't see them? Blame me if I missed it.
3. Consider working with fixtures to set up test data.

[dbdimitrov]

1 & 3 are fair points :); 2 is largely covered, but I agree maybe coverage can be increased a bit there