Reusable conch test server

Fixes #166 
Is *OPENSSH* the passphrase or a comment? Comment I think since this is a public key. It would be nice to add a comment explaining that and maybe use a keyword argument to make it clear what this key is used for.
 * ``_InMemoryPublicKeyChecker(public_key=key.toString("OPENSSH"))``
 * http://twistedmatrix.com/documents/current/api/twisted.conch.ssh.keys.Key.html#toString