Use mutation testing to improve test coverage

[cs-util]

The xUnit tests are now prepared in a way that should make them consistently pass or fail across arbitrary many runs. This now unlocks to finally use Stryker for mutation testing again. Running Stryker and then reviewing the findings is the next step here, if you are interested to learn about mutation testing in general this is the right task for you. And at the same time most likely you will find a few bugs that exist in cscore.