Clean up tests

[jagerber48]

#262 makes changes to the backend for some core functionality and also has some API changes. As a result it comes with decently large changes to the test suite. That is, some tests are actually changed.

However, many of the tests are written in ways that could be heavily modernized while still retaining the same test cases. I'm am restraining from doing much of this in #262 (though I am doing some). It would be beneficial to go through all tests and "clean them up". It would improve readability and make future maintenance easier.