testing: Migrate ikev2/host2host-transport-nat scenario to vici

This also restores the test as it was before the referenced commit so it
again, as written in the description, demonstrates that venus is unable
to ping sun without IPsec tunnel.

Fixes: f27fb58ae0 ("testing: Update description and test evaluation of host2host-transport-nat")

testing/tests/ikev2/host2host-transport-nat/posttest.dat

@@ -0,0 +1,6 @@
+alice::systemctl stop strongswan
+venus::systemctl stop strongswan
+sun::systemctl stop strongswan
+alice::iptables-restore < /etc/iptables.flush
+moon::iptables-restore < /etc/iptables.flush
+sun::iptables-restore < /etc/iptables.flush