CI: Add full matrix of runtime tests

After !263 (merged), we'll have most of our CI using tokio+native_tls, and one CI test that runs basic checks with async_std+rustls.

This is okay for a start, but we should instead really test everything with all combinations of our runtime implementations. Doing so will be expensive, so we should investigate which of these tests we want to add to our per-commit CI, and what we want to run in some kind of a nightly job.

The crates/tor-rtcompat/misc/matrix.py might be helpful, or we might wind up just replacing it.