Implement mutation tests #134
Description
Look into implementing mutation testing.
Assertj currently use https://pitest.org/ with their Maven projects which might be a good place to start.
This is mostly an investigation ticket, I have never set this up before or used mutation testing, so it should be interesting.
We then probably want a new job in the CI workflow to handle this, similar to the codestyle and license headers one. For now, we should allow it to fail until the API is stable.