tests/dns: Check you cannot add empty CNAME

tests/dns: Check you cannot add empty CNAME

This exercises the dns_check_name case in the DNS server. Directly
attempting to add an invalid name with leading . or double .. cannot be
done due to ndr_pull_component forcing the check on the client side
(leading to a CNAME name of NUL and unexpected data of the actual name).

Signed-off-by: Garming Sam <garming@catalyst.net.nz>
Pair-programmed-with: Bob Campbell <bobcampbell@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Garming Sam <garming@samba.org>
Autobuild-Date(master): Mon Dec 12 08:46:26 CET 2016 on sn-devel-144