Mark failing test as xfail for use of python-dns make_ds method
https://github.com/rthalley/dnspython/issues/343 documents broken use of
hashes in dns.dnssec.make_ds() and other python-dns methods. This is a
regression introduced with python-dns 1.16.
Mark the test as expecting to fail until python-dns is fixed in Fedora.
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>