Add a self-contained test suite

When I was cleaning up the code in preparation to open-source it, it was pointed out that we don't have a good test suite other than "does the monorepo still build," nor do we have a way to enforce that things are still isolating.

I think this is mostly going to be a shell script to exercise the isolation profiles and ensure they're isolated (e.g., you can't contact an outside service from inside net isolation or vice versa), and a little bit of code to smoke test the Makefile plugin and the C library directly via ctypes.

Probably tie it into GitHub Actions.