Support re-run failed tests only while asking bors-servo to retry

[CYBAI]

IIUC, back to the days we still host bors-servo on Mozilla's server, it supported only retrying failed tests if we do @bors-servo retry.

However, after moving to GitHub Actions, every retry will run all tests again no matter it succeeded or failed in previous job.

Thankfully, I just read GitHub announced partial re-run on GitHub Actions in this blog post and it says

Partial re-runs are available via our REST API and our command-line tools. Please check out the documentation for more detail.

Maybe we can try to see how we can improve bors-servo to only retry the failed jobs.

[CYBAI]

as we've fully migrated to run wpt test suites via GitHub Actions, I wonder this could be closed as resolved now? What do you think? (cc @mrobinson @sagudev)

[sagudev]

Well it is possible to rerun only failed jobs using GitHub UI, but it will run all tests in failed chunk.

[CYBAI]

but it will run all tests in failed chunk.

Good point. It doesn't retry "failed tests only".