Improve dnstap system test reliability

Improve dnstap system test reliability

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.