Tests should dump and validate TLS traces

[nbraud]

We have quite a few tests that produce actual TLS sessions:

• api.rs seems to test rustls against itself;
• many tests involve OpenSSL and rustls;
• rustls is tested against badssl.com;
• rustls is tested against a number of popular websites.

Each of those tests should dump the actual TLS session, ideally as a pcap file, and the session keys involved, so that the dumped trace can be automatically checked for protocol conformance.

The goal is to get stronger guarantees out of each test that gets written; as mentioned in #81, taking invalid transitions in the protocol is a major source of vulnerabilities.