Use PCAP to check tests

[cjpatton]

Checking whether a test has succeeded is currently a manual process that involves inspecting the endpoints' outputs. We will automate this by programmatically inspecting the PCAP of the handshake. This issue tracks that work.

[xvzcf]

Also, we use Wireshark for decoding the PCAPs, and it might not be able to completely do so when experimental TLS features are used, in which case we'd have to settle for checking whatever we can.

[cjpatton]

Is there a way to get Wireshark to decrypt handshake messages and output the raw data? We could parse it ourselves fairly easily ... what I mainly want Wireshark for is to avoid having to implement the key schedule and decryption logic.

[xvzcf]

When I last looked at it, it seemed like a custom dissector was the best way to go, but I'll revisit once we have the CI and runner working.