I am surprised this topic has not come up more often in the SEO forums - the topic of being able to test the Google Disavow Tool but uploading a file, waiting, removing links, waiting and repeating this process over and over again.
A Google Webmaster Help thread kind of aludes to it but not fully.
The issue webmasters are having is that they are disavowing a ton and waiting and not noticing an increase in rankings. They are then scratching their heads and asking, did I disavow links that were actually helping my site rank? So they go back in and remove some of the domains and URLs from the disavow tool and wait again.
You repeat this process over and over again until you see any ranking benefit. If you remove a URL and your rankings drop, add it back. Of course, doing this can take forever.
Google isn't dumb, they know SEOs and webmasters will test this.
In fact, at SMX Advanced, I believe Matt Cutts brought it up and said that is why they have a delay when processing the disavow tool. At least, that is one reason. It helps keep webmasters on their toes. They want webmasters to disavow links they think are not valuable and not links they think are just hurting them.
So they delay the process of the disavow to make things a bit foggy.
Forum discussion at Google Webmaster Help.