Use PIT for mutation testing

[gdejohn]

• Junit5 support hcoles/pitest#284
• https://github.com/pitest/pitest-junit5-plugin
• https://4comprehension.com/integrating-pit-mutation-testing-and-github-pages-with-github-actions/
• https://maarten.mulders.it/2020/06/mutation-testing-badge-with-pitest-and-stryker-dashboard/