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