[CI] Add mutation tests
This MR will give you a basic setup for mutation testing, unfortunately the version with the json log, isn't supported with PHP 7.2 so I need to install it directly in gitlab-ci.
Perhaps two different config files should be added to have one for locally and one for the CI, so that you also can run your tests locally.
I saw that the anybadge can take inputs to define the color based on the value.
Let me know what you think.