Add rekeying to vector tests

[mcginty]

We're just getting the new REKEY() method into snow, and it'd be nice to test it against cacophony.

Could possibly add a rekey_at_message in the main vector JSON structure that provides an index in the messages where a rekey happens, or a rekey key alongside payload and ciphertext in the message struct to indicate a rekey should be performed before decrypting.

Any opinion?

[centromere]

Thank you for pointing this out. I will start considering the best way to go about this.