Add test summary report to the merge request reports tab

What does this MR do and why?

Adds TestSummaryProvider, TestSummaryNavItem, TestSummaryPage and TestSummaryContent to the reports tab, matching the other reports. The summary, status icon and failed test logic moves into the test report widget utils.js so the widget and the report share it. The widget gets a View report button once the pipeline finishes.

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