First test would be to put the candy bar into a vacuum chamber and depressurize it until it fails.
It's possible that it might not rupture all the way to full vacuum. Some of those wrappers are pretty tough.
Knoshing on a candy bar after a flight might not be the best choice depending on location (melting notwithstanding). My OFFl rocket's nose weight was a 16oz. can of Diet Coke. Unfortunately for me, by the time I actually managed to launch it, and recover it intact for my L1 certification, the soda was warm. It was suggested that I should have used beer, but as I can't stand beer, I'd have needed to settle for an A&W, or Barq's. However, a candy bar, or too sweet of a soda, in a dry environment, and I'd only end up thirstier.