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/libipsec/host2host-cert/posttest.dat

@@ -1,6 +1,6 @@
 moon::swanctl --terminate --ike host-host 2> /dev/null
-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
 moon::sysctl --pattern net.ipv4.conf.all.rp_filter --system