Implement test coverage for MRs
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=499302) </details> <!--IssueSummary end--> We should support showing test coverage for diff lines in merge request Rapid Diffs. ![image](/uploads/1d2a1eaa8025cdfe0b2f150c3771b97b/image.png)
issue