Implement CI testing on GitHub pull requests

[maximecb]

Sorry to open another issue but one thing that seems to be missing is that there are no automated tests that run when a pull request is opened. I understand that yjit-metrics is a complex system with multiple machines involved, but it should be possible to run some components of that locally to test at least part of the system. This isn't necessarily super high priority, but something that we should have eventually.

[rwstauner]

Yeah, that made me sad, too, we can get something setup

[rwstauner]

closed by #382