Test for revival to a host behind NAT.

This uses auto=keep on east, meaning once road has initiated, east
wants the tunnel to remain up. Road then sends a delete to emulate
an idle connection being torn down. Normally, east cannot initiate to the
host behind NAT to keep the connection up. But if it is quick, it can
re-use the existing NAT mapping to establish a new tunnel.

In this case, we really make road disappear, so that the revival fails.
East should then still cleanup and delete the instance.
