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/gcrypt-ikev1/alg-serpent/posttest.dat

@@ -1,2 +1,2 @@
-carol::systemctl stop strongswan-swanctl
-moon::systemctl stop strongswan-swanctl
+carol::systemctl stop strongswan
+moon::systemctl stop strongswan