Lower the minimum expected dnstap output file size

Lower the size requirement for the dnstap output file produced during
the "dnstap" system test from 454 to 450 bytes; while files of that size
are not generated in any GitLab CI job, they are in other environments
where the test passes.

bin/tests/system/dnstap/tests.sh

@@ -569,7 +569,7 @@ EOF
 
 	echo_i "checking unix socket message counts"
 	sleep 2
-	retry_quiet 5 dnstap_data_ready $fstrm_capture_pid dnstap.out 454 || {
+	retry_quiet 5 dnstap_data_ready $fstrm_capture_pid dnstap.out 450 || {
 		echo_i "dnstap output file smaller than expected"
 		ret=1
 	}