testing: Add ikev2/start-action-start scenario

This tests the behavior for configs with start_action=start during
reloads of the config (updates/removal).

testing/tests/ikev2/start-action-start/posttest.dat

@@ -0,0 +1,5 @@
+sun::systemctl stop strongswan
+moon::systemctl stop strongswan
+moon::iptables-restore < /etc/iptables.flush
+sun::iptables-restore < /etc/iptables.flush
+moon::rm -rf /etc/swanctl/conf.d/