Basic pluto with IKEv2 on the initiator (west).

This test does not actually do IKEv2, it just *tries*.
A packet filter discards the first 6 IKEv2 packets, at which point it falls
back to trying IKEv1, which should succeed.

The responder will indicate with a notify that it could have done IKEv2,
and as a result, the initiator will immediately replace the SA with an IKEv2
SA.

This test could be extended to drop more packets --- and a second test should
be created that considers what happens if the IKEv2 fails to get established.