ci: Add /retest option to rerun only failed test

[learnitall]

To help address cases where flaky test are blocking a PR, we should add a /retest command which only reruns failed tests. This is a better alternative to consistently using /test which reruns all tests, introducing the probability to come across new flakes.

[brlbil]

I am working on an implementation that runs tests as normal with the /test command and re-runs only the failed test if the /test command is issued again. If the commit SHA changes the /test runs all the tests again.

[brlbil]

This future is implemented and in use now.