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

This prevents failures due to remaining conntrack entries.

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

@@ -5,4 +5,3 @@ alice::iptables-restore < /etc/iptables.flush
 venus::iptables-restore < /etc/iptables.flush
 sun::iptables-restore < /etc/iptables.flush
 moon::iptables -t nat -F
-moon::conntrack -F