ipatests: remove xfail from test_dnssec
    
    The nightly test test_dnssec.py::TestInstallDNSSECFirst::test_chain_of_trust
    used to fail because of https://github.com/rthalley/dnspython/issues/343,
    but the issue has been fixed upstream and does not happen any more since
    PRCI is using python3-dns-1.16.0-7.
    
    Remove the xfail.
    
    Reviewed-By: Mohammad Rizwan <myusuf@redhat.com>