server/tests.at, t/keyring-*: Rename test keyrings.

server/tests.at, t/keyring-*: Rename test keyrings.

We're going to be playing games with these which will change which
groups they use, so naming them after the groups is silly.

Also switch the prime group to use a smaller field (catacomb-ll-160-1024
specifically) because the primality checking takes ages and we don't
actually need any security.