Release Please Bot should trigger CI

[JustinBeckwith]

I was looking at this PR created by the release-please bot:
googleapis/sloth#328

And noticed that it didn't trigger a kokoro run. I'm not sure what Kokoro uses to decide if jobs should be run, but we should find a way to have PRs created by our bots automagically trigger CI if applicable.

[chingor13]

I think a trusted-tester bot could work for this. The repository would maintain a .github/trusted-tester.yml with a list of approved users.

On pull_request.opened or pull_request.synchronized the bot would add the kokoro:force-run label to the PR.

@crwilcox

[JustinBeckwith]

The problem with that approach is that we would need to re-trigger CI every time there is an update to master. Renovate does a good job of auto-updating it's own PRs (which we should consider regardless).

[bcoe]

this is becoming a bit of a hassle, specifically for release-please bot, it would be nice to look into whether we can at least get that bot whitelisted, such that tests kick off whenever it opens a PR.

[JustinBeckwith]

Happy to say this is working swell now ;)