Summary of testsuite health page

[stianst]

Remove Flaky tests, and Recently failed jobs from Overview.

Add Tests table to Overview page with the following columns:

• Number of unreported PR retries last 30 days (Retried PRs last 30 days without Resolved by link)
• Number of reported PR retries last 30 days (Retried PRs last 30 days with Resolved by link, where issue is not closed)
• Number of slow PR runs last 30 days (Slower than 200 minutes)
• Number of unreported failed jobs (failed jobs without resolved by link)
• Number of reported failed jobs (failed jobs with resolved by link, where issue is not closed)
• Number of unassigned open flaky tests
• Number of assigned open flaky tests
• Open issues not resolved within 7 days
• Per-team open issues