Just a quick note: I failed for open DNS servers, and so I emailed my host about it. They assured me that the DNS servers are not open and <insert techy jargon here> something else is triggering the test to show a fail result.
If you failed, then the server where your site is hosted is open for DNS recursion. I don't think there is a way for that report to return false information although I could be wrong. If that is the case, we need to alert Scott at the DNS Report so they can investigate.