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/multi-level-ca/pretest.dat

@@ -1,6 +1,6 @@
-moon::systemctl start strongswan-swanctl
-carol::systemctl start strongswan-swanctl
-dave::systemctl start strongswan-swanctl
+moon::systemctl start strongswan
+carol::systemctl start strongswan
+dave::systemctl start strongswan
 moon::expect-connection research
 carol::expect-connection alice
 carol::swanctl --initiate --child alice 2> /dev/null