I don't really know the answer to that. I am pretty sure they mean a single test forum. The intent, I believe, was to allow you to practice an upgrade on your site prior to doing it on the live site, or test modifications out prior to putting them on your live site. I don't think they ever thought about the developer who may want to develop a modification to work on several different versions (or the moderator who may want to have different versions to be able to help better

).