We're all nervous when we test. If you care about what you are doing it's very natural to be nervous.
In Tang Soo Do, I have seen that our Sa Ba Nim is not too hard on the white belts. It's probably the same in TKD.
Be sure to practice daily - it's better to not practice everything daily. Say, do kicks one day, forms the next. At night, visualize your forms, kicks, self-defense, etc., before going to sleep.
I'd advise you to not wait the day of the test to begin practicing. Some people do that and create unnecessary stress for themselves and their classmates as well.
I know people say relax. I don't know of a way to relax. How do you relax? Being confident of your material is the best you can do. Also be cognizant of the fact that if you do not pass, that is not failure. It just means you weren't ready to test.
Good luck to you. I am sure you really will do fine.