s3: spoolss: Extend publish_toggle test to check returned GUID string format

s3: spoolss: Extend publish_toggle test to check returned GUID string format

Extend the rpc.spoolss.printer.addprinter.publish_toggle test to
check the format of the returned GUID string in GetPrinter info
level 7 structure.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=12993

Signed-off-by: Samuel Cabrero <scabrero@suse.de>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: David Disseldorp <ddiss@suse.de>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Wed Oct 11 06:39:00 CEST 2017 on sn-devel-144