Test anixety is something that I think we all have at times, some worse than others.  The easiest eay to overcome it is to be prepaired and make sure you have good study habits.  I think this article gave some good tips in overcoming test anxiety.  Always communicate with your instuctors and make sure that they know what you are going through.  There have been times when I have asked to sit in the hallway just because I do better with zero distractions.