But you aren't taking the LSAT until September? That gives you plenty of time, even if you can only devote a few hours a week.
Plus, if, as you say, you don't need to pass this other test, and the fact that you didn't pass it (if you don't) won't end up on a transcript or anything...I guess it doesn't matter.
I think you'll be the only person who will be able to answer this because you will be the best gague of how much LSAT work you need to do. When is this other test? If it's well before September (2 or 3 months or more), I'd say you can devote some of your LSAT time to it, because as someone said above...September is a long ways off.