testing: Run 'conntrack -F' before all test scenarios

This prevents failures due to remaining conntrack entries.

testing/tests/ikev2/compress-nat/posttest.dat

@@ -5,6 +5,4 @@ alice::iptables-restore < /etc/iptables.flush
 carol::iptables-restore < /etc/iptables.flush
 bob::iptables-restore < /etc/iptables.flush
 moon::iptables-restore < /etc/iptables.flush
-sun::iptables-restore < /etc/iptables.flush
-moon::conntrack -F
-sun::conntrack -F
+sun::iptables-restore < /etc/iptables.flush