Add osclientcerts tests

[JeremyRand]

osclientcerts is the Firefox PKCS#11 module that reflects client certificates from the Windows and macOS OS-wide cert store. We should add tests for it, by putting it behind pkcs11proxy and p11proxy and making sure the behavior doesn't change.

[JeremyRand]

Note that osclientcerts is written in Rust, so there's a decent chance it will trigger some novel code paths for us.