cups: Fix printing tests

It seems like there's an upstream bug in the "lpstat" command. We need
to specify the server's port.

Further information: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=711327

[root@client:~]# lpstat -H
/var/run/cups/cups.sock

[root@client:~]# lpstat -h server -H
/var/run/cups/cups.sock:631

[root@client:~]# CUPS_SERVER=server lpstat -H
server:631

[root@client:~]# lpstat -h server:631 -H
server:631

nixos/tests/printing.nix

···
 
       # Do some status checks.
       $client->succeed("lpstat -a") =~ /DeskjetRemote accepting requests/ or die;
-      $client->succeed("lpstat -h server -a") =~ /DeskjetLocal accepting requests/ or die;
+      $client->succeed("lpstat -h server:631 -a") =~ /DeskjetLocal accepting requests/ or die;
       $client->succeed("cupsdisable DeskjetRemote");
       $client->succeed("lpq") =~ /DeskjetRemote is not ready.*no entries/s or die;
       $client->succeed("cupsenable DeskjetRemote");