[client] re-use encryption/HMAC keys in --test mode

The client --test mode decrypts SPA packet data as a final step, but get_keys()
was being called to re-acquire the encryption/HMAC keys.  This commit reuses
the same keys that were supplied for SPA packet encryption/authentication
because the most important code to test is not get_keys() but rather libfko
encryption/decryption/authentication operations.
[server] minor --help update to include cipherdyne.org URL