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.

bin/tests/system/dnstap/tests.sh

@@ -669,6 +669,7 @@ EOF
 	$FSTRM_CAPTURE -t protobuf:dnstap.Dnstap -u ns4/dnstap.out \
 		-w dnstap.out > fstrm_capture.out 2>&1 &
 	fstrm_capture_pid=$!
+	sleep 1
 	$RNDCCMD -s 10.53.0.4 dnstap -reopen | sed 's/^/ns4 /' | cat_i
 	$DIG $DIGOPTS @10.53.0.4 a.example > dig.out