Ahhh.... oh.. I don't know. I've never been to any kind of martial arts tests (can't afford to go to classes *cries*) so I can't relate to the same kind of nervousness.
However, before big exams, I study hard. Its the only way to be sure that I'm doing my absolute best. Having the knowledge that I cannot possibly do any better than I am at that moment calms my nerves.
I also kind of go cold. Before my exam, I change. Its wierd. Some days I can barely remember the exam. But what happens is, I turn into a complete robot. My emotions just run dry. I think of nothing but here, now, and the exam. I just cease to think in a normal way. I don't feel anything.
Its not voluntary either, but it helps.
Good luck with the test (and if its passed already I hope you did well)