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/rw-eap-sim-only-radius/posttest.dat

@@ -1,6 +1,6 @@
-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
 alice::killall freeradius
 moon::iptables-restore < /etc/iptables.flush
 carol::iptables-restore < /etc/iptables.flush