Improve JUnit test reports in merge request widgets

What does this MR do?

This MR does the following things

• Invalidate MergeRequest#compare_test_reports cached results (via ReactiveCache), when pipeline status is updated
• Change the lifetime of cached results to 10 minutes.
• Ci::Pipeline#test_reports corrects test reports from the latest builds
• Add feature spec for JUnit test reports on merge request widgets

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a Backend maintainer
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Close: https://gitlab.com/gitlab-org/gitlab-ce/issues/49966