testing: Enable firewall for ikev2/compress scenario

Additionally, send a regular (small) ping as the kernel does not
compress small packets and handles those differently inbound.

testing/tests/ikev2/compress/posttest.dat

@@ -1,2 +1,4 @@
 moon::ipsec stop
 carol::ipsec stop
+moon::iptables-restore < /etc/iptables.flush
+carol::iptables-restore < /etc/iptables.flush