As a former professional musician turned pre-med, anxiety before a performance/audition (or in this case a test) is something that I grappled with often. I am absolutely sure that the #1 greatest cause of anxiety is incomplete preparation. Did you go over the material until you could recite it in your sleep? Did you exhaust every single resource available to you before the test? Often the answer is no, and when the test jitters come, we look for the answer about how to control testing anxiety. But really this is treating the symptom, not the cause. In your brain, you will know going into the test that there was more you could have done, and it will ramp up your nerves exponentially.
Of course, this is not always true, but years of experience have taught me that it's a pretty good place to start.
Another strategy I use to ward off testing anxiety is to channel the energy positively. Sound strange? Bear with me for a moment....
Coming into any kind of situation in which your skills are being evaluated for an important result, you will naturally build up a lot of energy. If you're not careful, this can turn into anxiety/neurosis. But you can also use this extra boost of energy to your advantage. This is one of the reasons I've never enjoyed performances on beta-blockers: they always seem flat, dull compared to the brilliance they might have with that extra wave of energy behind them.
So how can you do this? My trick is to get really excited about the exam material. I'm a bit of a nerd, so this was never too difficult for science exams. Really look forward to solving some problems, showing off what you know, and getting the opportunity to see what could potentially be some cool new material. I remember right before my MCAT, the thoughts in my head were mostly that I wanted to get started on the test to see all of the interesting information in the science and reading passages. During Orgo tests, I was anxious to see what kinds of cool syntheses and mechanisms we would get to solve.