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/sql/net2net-psk/posttest.dat

@@ -1,4 +1,4 @@
-moon::systemctl stop strongswan-swanctl
-sun::systemctl stop strongswan-swanctl
+moon::systemctl stop strongswan
+sun::systemctl stop strongswan
 moon::iptables-restore < /etc/iptables.flush
 sun::iptables-restore < /etc/iptables.flush