Hook up to merge bot

[jlebon]

We should hook up homu to this repo. The issue with "Squash and merge" is that you lose the semantics of separate commits and have a non-sensical commit msg that joins all the msgs. Commit msgs from fixup! commits also get in, which is yucky.

Let's install a basic testsuite that homu can gate on (I'm ignoring the existing PR tests we have now since as I mentioned before, it's conceptually not really appropriate for gating PRs). And from there, hook up homu.

[miabbott]

Homu all the things!

Not opposed to this, but would like input on ideas on how to gate on PRs. We were working toward something a little more intelligent in our PAPR tests, but still have work to do in that space.