fix: test: Check read value in upforwd/ans4

The ans4 server wasn't shutting down cleanly sometimes.  Check that read returns the
expected value for the message length or exit read loop.

Closes #4301

Merge branch '4301-check-read-value-in-ans4' into 'main'

See merge request isc-projects/bind9!9537

bin/tests/system/upforwd/ans4/ans.pl

@@ -357,7 +357,9 @@ for (;;) {
 			last unless $n == 2;
 			my $len = unpack("n", $lenbuf);
 			$n = $conn->sysread($buf, $len);
+			last unless $n == $len;
 		}
 		sleep(1);
+		$conn->close;
 	}
 }