Update nsupdate test

The nsupdate system test did not record failures from the
'update_test.pl' Perl script. This was because the 'ret' value was
not being saved outside the '{ $PERL ... || ret=1 } cat_i' scope.

Change this piece to store the output in a separate file and then
cat its contents. Now the 'ret' value is being saved.

Also record failures in 'update_test.pl' if sending the update
failed.

Add missing 'n' incrementals to 'nsupdate/test.sh' to keep track of
test numbers.

bin/tests/system/nsupdate/update_test.pl

@@ -58,7 +58,7 @@ sub assert {
     my ($cond, $explanation) = @_;
     if (!$cond) {
 	print "Test Failed: $explanation ***\n";
-	$failures++
+	$failures++;
     }
 }
 
@@ -79,6 +79,7 @@ sub test {
         assert($rcode eq $expected, "expected $expected, got $rcode");
     } else {
 	print "Update failed: ", $res->errorstring, "\n";
+	$failures++;
     }
 }