Resolve "missing messages in `dnstap` system test after query failure"
The dnstap system test fails intermittently, and it appears to be
a timing issue - adding a short delay after running fstrm_capture,
and before running dnstap -reopen improves the situation from
50% failures (5 out of 10 times) to 0% failures (0 out of 20 times),
tested locally.
The reason is that fstrm_capture is executed in the background,
and due to OS scheduling and other factors, the listener socket
may not be ready when the following command runs and tells named
to (re)open it.
Closes #3924 (closed)