Basic pluto with IKEv2 using PSK on the initiator (west), and on the responder.

This test uses aes_gcm-sha2_512 for IKEv2, and checks that the nonce is
large enough. This is a port of downstream test for rhbz#1297816

Math looks a little weird because on emit we get the nonce size going in
the nonce payload but on parse, we only get the nonce payload size and
have to subtract the 4 bytes of the nonce header.