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/nat-rw/posttest.dat

@@ -1,6 +1,6 @@
-sun::systemctl stop strongswan-swanctl
-alice::systemctl stop strongswan-swanctl
-venus::systemctl stop strongswan-swanctl
+sun::systemctl stop strongswan
+alice::systemctl stop strongswan
+venus::systemctl stop strongswan
 alice::iptables-restore < /etc/iptables.flush
 venus::iptables-restore < /etc/iptables.flush
 sun::iptables-restore < /etc/iptables.flush