Cross-check certificate generation

@Lastor identified an interoperability issue between Sequoia and rPGP. He reports that an RFC 9580 certificate generated by Sequoia is not usable by rPGP. I took a quick look at the interoperability test suite and it appears that the tests use fixed artifacts (e.g., this one). I think it would be helpful to have each implementation generate an artifact and test whether other implementations are able to consume them.