testing: Use renamed systemd unit

While the alias is available after enabling the unit, we don't
actually do that in our testing environment (adding a symlink manually
would work too, then again, why not just use the proper name?).

testing/tests/swanctl/net2net-start/pretest.dat

@@ -1,5 +1,5 @@
 sun::iptables-restore < /etc/iptables.rules
 moon::iptables-restore < /etc/iptables.rules
-sun::systemctl start strongswan-swanctl
-moon::systemctl start strongswan-swanctl
+sun::systemctl start strongswan
+moon::systemctl start strongswan
 moon::sleep 0.5