To begin with - you definitely have the right attitude toward your testing. You're nervous - and that means you're taking it seriously, which you should do.
Everyone makes mistakes - and you will too. How you respond to your mistakes will be as important as anything else. Take them in stride.
Bring water and/or sports drinks, and something to eat that is quick, not messy, and highly digestible - energy bars are good, as are fresh fruit (already chopped/peeled/etc.), granola bars, or anything you would take hiking that requires no preparation. If you don't get to grab a snack during testing (likely) you'll need it when you're done.
The people who will be testing you have seen other students your rank test, and it's highly unlikely you will surprise them. Do you best to demonstrate that you already are a black belt. Passing such a test is by no means guaranteed - but no good instructor will allow you test if you aren't capable of passing. You're going to have to work - and work hard - for your belt, but you have the potential, or you wouldn't be put on the floor to test.
Good luck, and let us know how it goes!