In the tests, don't start agetty on /dev/ttyS0

Running agetty on ttyS0 interferes with the backdoor, which uses ttyS0
as its standard error.  After agetty starts, writes to the stderr file
descriptor will return EIO (though doing "exec 2>/proc/self/fd/2" will
miracuously fix this).

http://hydra.nixos.org/build/3252782

tests/nat.nix

@@ -40,6 +40,7 @@
       startAll;
 
       # The router should have access to the server.
+      $server->waitForUnit("network.target");
       $server->waitForUnit("httpd");
       $router->waitForUnit("network.target");
       $router->succeed("curl --fail http://server/ >&2");
@@ -68,7 +69,7 @@
       $client->fail("ping -c 1 server >&2");
 
       # And make sure that restarting the NAT job works.
-      $router->succeed("start nat");
+      $router->succeed("systemctl start nat");
       $client->succeed("curl --fail http://server/ >&2");
       $client->succeed("ping -c 1 server >&2");
     '';