Where is the test suite?

How do I run buck2's self test suite? I don't see it anywhere in the open source repo, and there's no documentation about running tests that I can find. If I make a change to prelude, or core buck2 runtime, or whatever else, I would like to be able to validate my changes before making a PR.

Is this possible? I would expect a top-level folder in the repo called `tests` or something. But maybe I'm just not finding it?