Improved 'already processed' message

Change-Id: I3e5ccf20335ac7b262b9a73ae7351c70ab92a972
diff --git a/t/batch_file.t b/t/batch_file.t
index 50b59a8..ccfa62f 100644
--- a/t/batch_file.t
+++ b/t/batch_file.t
@@ -117,7 +117,7 @@
 # Check overwriting
 $bf->{overwrite} = 0;
 
-ok(!$bf->process($path => $output), 'Process file');
+is($bf->process($path => $output), -1, 'Process file');
 
 done_testing;
 __END__