yeah, you can have a snack during your break - you just need to go into the hallway. i would suggest an apple or PowerBar - good brain boosters. eat a banana in the morning, but at least an hour before the test because they're nutritionally dense and take a while to digest. that could make you a bit sluggish at first, but about an hour after you eat it, it's great brain food.
definitely drink some water or Gatorade (NOT Red Bull - it will make you jittery as hell) during the breaks, but not too much because you don't want to get distracted by your bladder!!
i don't remember if they call you back from breaks - i kept a real close eye on the clock myself. you literally have just enough time to take a sip of water, go to the bathroom, and inhale a snack.