testing: Add DSCP scenario that uses simple labels

Instead of creating two IKE_SAs with different identities, this scenario
uses simple labels to select the correct child config.

testing/tests/ikev2/net2net-dscp/posttest.dat

@@ -0,0 +1,8 @@
+moon::systemctl stop strongswan
+sun::systemctl stop strongswan
+moon::iptables-restore < /etc/iptables.flush
+sun::iptables-restore < /etc/iptables.flush
+alice::iptables -t mangle -F OUTPUT
+venus::iptables -t mangle -F OUTPUT
+bob::iptables -t mangle -F OUTPUT
+