testing: Add ikev2/mobike-virtual-ip-nat scenario

This tests moving from a public IP behind a NAT and back (with proper
changes of the UDP encapsulation).

testing/tests/ikev2/mobike-virtual-ip-nat/posttest.dat

@@ -0,0 +1,6 @@
+alice::ipsec stop
+sun::ipsec stop
+alice::ifdown eth1
+alice::iptables-restore < /etc/iptables.flush
+sun::iptables-restore < /etc/iptables.flush
+moon::iptables -t nat -F