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/host2host-cert/pretest.dat

@@ -1,7 +1,7 @@
 moon::iptables-restore < /etc/iptables.rules
 sun::iptables-restore < /etc/iptables.rules
-moon::systemctl start strongswan-swanctl
-sun::systemctl start strongswan-swanctl
+moon::systemctl start strongswan
+sun::systemctl start strongswan
 moon::expect-connection host-host
 sun::expect-connection host-hhost
 moon::swanctl --initiate --child host-host 2> /dev/null