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/tnc/tnccs-20-mutual-pt-tls/posttest.dat

@@ -1 +1 @@
-sun::systemctl stop strongswan-swanctl
+sun::systemctl stop strongswan

testing/tests/tnc/tnccs-20-mutual-pt-tls/pretest.dat

@@ -1,4 +1,4 @@
-sun::systemctl start strongswan-swanctl
+sun::systemctl start strongswan
 moon::cat /etc/pts/options
 moon::sleep 1
 moon::/usr/local/bin/pt-tls-client --optionsfrom /etc/pts/options