Refactor TestReports to use Dynamic Fixtures

Summary

Refactor the mock_data introduced as part of !18255 (merged) to use dynamic fixtures as suggested in this rfc: gitlab-org/frontend/rfcs#13 (closed).

Improvements

• Provides more reliable fixture usage
• Tests should break when backend changes and not silently pass with old mock_data

Risks

• Requires ruby code to be written, possibly by frontend

Involved components

• spec > frontend > pipelines > test_reports > *