Implement part of #1557: Add tests for replayRate controller.

[rahat2134]

Proof

Checklist

• [X] A link to the issue/user story that the pull request relates to
• [ ] How to recreate the problem without the fix
• [ ] Design of the fix
• [ ] How to prove that the fix works
• [ ] Automated tests that prove the fix keeps on working
• [ ] Documentation - any JSDoc, website, or Stackoverflow answers?

Issue/User story

Add tests for replayRate controller.