Preworkouts don't do a whole lot for you if the body isn't able to process them and utilize them. You can take whatever preworkout you want, but if your body is low on glycogen, hydration and electrolytes, you're not going to get much benefit. Think of it like a car- Preworkout is basically just warming up the engine before going out on the track. You still need gas to drive (energy), and mechanical components to remain competitive (conditioning/fitness).
You're better off to start hydrating about two days out and keep hydrating up until the start of the test. Start eating better foods a couple days out as well if you don't have the best diet. Avoid excess fat, and try to avoid too much processed food. Eat a lot of complex carbs like sweet potatoes, long grain rice, yogurt and whatnot. The day before, do some serious carbo-loading. Pasta is especially popular for this.
Then, during your APFT, keep sipping a sports drink. I have my own intra-workout drink I make myself for the gym, but you can really use any decent sports drink like Gatorade or Powerade.
Also, make sure you get a LOT of sleep before hand. At least the two previous nights if possible. Sleep and hydration will do far more than any preworkout. In all my years in the Army, and then now with my lifting, I would go out on a limb and say that a majority of people are not properly hydrating.
If you have all that stuff covered, then you can start looking at a preworkout supplement to "prime" everything up.
I would recommend that you watch this video by Will Brink. He's a personal fitness SME, and he's pretty heavily invovled in the tactical community with relation to fitness.