Run mutation tests for any new tests added by each PR in CI

[jamescrosswell]

Mutation Testing could cover such issues early ... e.g. if we only run mutation tests on the changes per PR, and only via the test cases that cover the /src/ changes of the PR.

Originally posted by @Flash0ver in #5337

References

• Microsoft docs - Mutation Testing
• Stryker Mutator - Introduction

[linear-code]

NET-721

[Flash0ver]

Also has blog post & S'n'T potential ... considering #5337 (comment).

As discussed with @jamescrosswell, it should neither impact the stability nor the duration of CI workflows triggered by a Pull Request.