Generalise test compare service

What does this MR do?

It adds a base class for CompareTestReportsService containing common code with CompareLicenseManagementReportsService which is present in GitLab Enterprise Edition.

This MR is the CE counterpart to https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8032

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/7788

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [ ] Conforms to the database guides
• ~~[ ] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.~~