WIP: Feature specs for Test reports on merge request with FE and BE code

What does this MR do?

This MR provides the following things

• Feature specs for Test reports on merge request
• the unified branch for QA/FA to the feature JUnit XML Test Summary In MR widget

FYI, we're merging FE and BE branches separately. https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20936, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20576.

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a Backend maintainer

What are the relevant issue numbers?

Related: https://gitlab.com/gitlab-org/gitlab-ce/issues/45318