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/ip-split-pools-db-restart/posttest.dat

@@ -1,5 +1,5 @@
 carol::swanctl --terminate --ike home
 dave::swanctl --terminate --ike home
-carol::systemctl stop strongswan-swanctl
-dave::systemctl stop strongswan-swanctl
-moon::systemctl stop strongswan-swanctl
+carol::systemctl stop strongswan
+dave::systemctl stop strongswan
+moon::systemctl stop strongswan