Regression detection follow-ons

[woodruffw]

Once #201 is merged, some things:

• Leaving a new comment each time is pretty noisy, we could modify a single comment instead.
• We should gate a PR on no-regressions || (regressions && regressions-triaged) labels. I tried this with a basic workflow in Detect testcase regressions #201, but it's limited by the fact that workflows can't trigger other workflows.

[alex]

Suggestion: Comment on all failures, and only comment on passes if it's going from fail -> pass.

[woodruffw]

Suggestion: Comment on all failures, and only comment on passes if it's going from fail -> pass.

Thanks, done with 88f27df