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