tests: introduce mutation tests

[jkowalleck]

introduce mutation tests / test mutations

products:

• https://infection.github.io/
• ... to be continued

acc / crit

• have mutation testing avialable -- via Introduce mutation testing tool: infection #129
• have mutation testing as part of CI/CT in github pipeline
• have mutation testing as part of CI/CT in composer test suite
• have mutation testing passing (no escapes and so on )