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

This test uses AES-GCM, using esp=aes_gcm_c-256-null, where a/b/c means
ICV lengths 8, 12 and 16, and 256 is the AES key length (and the null
needs to be there to prevent Libreswan from sticking in an additional
auth algorithm).