'm assuming you can take drinks (and perhaps choccy) into the aptitude test?
I very much doubt it - you should assume that you can't take anything in. Imagine if a candidate split water onto a keyboard/other device, wrecked it, and insisted on being re-tested?
but as doing a bleep test with my mobile in my pocket would be a bit strange
I expect it will be possible to put it down inside the gym, in a container with other people's phones, wallets etc.
Suggest loading up on the carbohydrates the night before - will give you plenty of slow release energy for the day ahead.