[pulsarbot] Add option to re-run all tests (even if they succeeded)

[devinbost]

Purpose

When testing for flaky tests, it is often useful to be able to trigger all tests to be re-run.

Context

In the past, when I wanted to get all the tests to run again, I would make an arbitrary change to a comment in one of the code files to trigger a new build. However, this has not been working recently. (Instead, the builds all just automatically pass instead of completely running again.)

New Feature

Currently, pulsarbot is capable of triggering failed tests to be re-run, but it doesn't currently support an option to re-run tests that passed.
Please add this support to the pulsarbot that listens to comments in the PRs to trigger build actions.

@sijie This one is for you. :)