As ikev2-algo-04-aes-gcm but now using 128bit keys

This test uses AES-GCM, using esp=aes_gcm_c-128-null, where a/b/c means
ICV lengths 8, 12 and 16, and 128 is the AES key length (and the null
needs to be there to prevent Libreswan from sticking in an additional
auth algorithm).