server/tests.at, t/keyring-*: New tests for key management.

server/tests.at, t/keyring-*: New tests for key management.

Ensure that everything works when different peer associations use
different private keys, and when the keys, algorithms and groups change
under our feet.

This involves adding a new peer to the keyrings, and inventing a new
keyring.  More interestingly, it also involves plumbing together a
network of three peers, which is where the earlier refactoring of the
test communications machinery pays off.