Hi, sweetheart,
For one thing, periods are lots more uncomfortable when you have to pee, so although you should always stay hydrated, if you stop drinking fluids about three hours before the test and pee right before the test, that will help you feel less bulked up or pressured in the lower abdomen.
If nobody has told you yet how much more comfortable ibuprophen makes you (that is, Nuprin or Advil) when you have a period, try it. It is *amazing* what a difference it makes. It is for smoothing out clenched muscles, which of course is exactly what periods are, the uterus contracting and releasing to let the lining go.
If you think there is ANY chance you will be dealing with leakage, just be super sure to change your (whatever you use, pad or tampon) right before the test when you go to the bathroom. If you use tampons, you could add a pad, just as a doubly safe precaution, only because then you won't be thinking about it.
Take iron this weekend. It helps you to have built up your iron supply any month, women lose some in their endometrium when it sheds every month. Don't take so much that you get constipated (I always take mine along with a fiber gummy, to keep me regular) but do take some. You can take it with vitamin C for good absorption.
Lots of women have done amazing things when also dealing with something only women deal with, not just taking tests successfully. Your brain is not having a period! :) Women's bodies are designed to do the cool things they do that men can't do, such as have babies, while continuing to live one's daily life. We aren't defined by the physical side or the reproductive side, we are SO MUCH MORE.
Now, about tests. Don't freak yourself out. You will go in and you'll do your best. You'll stay focused, you won't fret. It is a test, not a crystal ball about your worth. Simply remember that you have all the information in your head that you need (already in there) to show on a test what you know. It will be there for you as you let the questions carry you along.
Good luck, you'll come out fine.