They do not test them at all, as far as I can tell. Their security is at best laughable, and they have a pretty rotten track record regarding frequency of puzzling assertions spewing out of them (which the famous Liberal Media seems to miss entirely), and there is flat-out no auditing method for them that isn't essentially asking them to repeat what they just said. "The machine says McCain won." "Can you get an audit for that?" "I did the SQL query manually, and it says the same thing." "Good enough for me!"

And, as I expect you've seen, xkcd makes a reasonable objection to the machines' basic design philosophy.