Disable dnstap testing globally

Disable dnstap testing globally

The dnstap system test fails permanently. Given that we don't have the
resources to investigate and fix the issue in this EoL branch, turn off
dnstap globally.

    I:dnstap:checking that named-checkconf detects no error in good-fstrm-set-reopen-interval.conf
    I:dnstap:wait for servers to finish loading
    I:dnstap:checking initial message counts
    I:dnstap:checking UDP message counts
    I:dnstap:ns3        3 expected 4
    I:dnstap:failed
    I:dnstap:checking TCP message counts
    I:dnstap:checking AUTH_QUERY message counts
    I:dnstap:checking AUTH_RESPONSE message counts
    I:dnstap:checking CLIENT_QUERY message counts
    I:dnstap:checking CLIENT_RESPONSE message counts
    I:dnstap:ns3        1 expected 2
    I:dnstap:failed

But enable dnstap in Debian "stretch" CI jobs to ensure it builds and
gets at least minimal testing.