Brent is right, you can only bring tissues in case you sneeze and your id into the test center and not too much else.

Bring three snacks for tomorrow. One for when you are traveling there and/or while you are waiting to go into the test, and one for each of the two breaks. The snack for before the test can be more substantial like that sandwich you mention. The snacks for the breaks should actually have a higher sugar content so those biscuits (or cookies) you mention are a good option.

(As an American I think that "biscuits" is such a better word. "Cookies" is not a very elegant for something so delicious. Also I love the word "Crisps" for potato chips.)

Do contact the test center if you have any doubts. If you do not get the chance to contact them (test is tomorrow, right?) then bring something that is well sealed up and you can leave it in a bag or jacket pocket or something. A couple of bags of biscuits would serve nicely.

Whatever you do, bring a snack! And bring a little bit or water or another drink as well. No one should try to do this 3.5 hour test without a couple of snacks and a least a sip or two or water!

Best of luck tomorrow!!