What would be the best snack to eat prior to taking the APFT?

Not dairy products.

A banana - half of one or the whole thing depending on your appetite. A half banana should good approximately 20-30 minutes before. It will provide you with the potassium and nutrients you need. If you want, you can put a little bit of PB or almond butter. If it is over an hour of exercise, perhaps have a greek yogurt and hour before.

I usually throw in a dip of Copenhagen and slam a cup of coffee.