Well, I know exactly what you are going through. I am a court reporter. In order to work in the state I live in we have to pass a certification test at 225 WPM. I have THE WORST test anxiety in the world. I literally took my certification test at least 15 times, no kidding. Every time I'd take it I'd sweat and get all nervous. If I thought I was doing well I'd trip myself up and screw up. A lot of my classmates had the same problem.
So I can totally relate to the stress you feel. Some people I knew took Xanax or a beta blocker called Inderal to help with nerves or had a drink. I ended up passing without any meds though. But I just had to keep at it. So just go in there and try not to worry about it too much and let it come naturally. I know it's easier said than done, but if you happen not to pass, just keep trying.
And most importantly, good luck!