When my kids express their concerns about failing a test I ask them to stop talking, take a deep breath and ask them to recall a test they took 2 years ago in the second week of November. Most of the time my kids will look at the ground, ponder for a moment or two and respond with "I don't remember". Then I ask them if they feel they have had a successful academic career to which they answer "yes".
The point I'm trying to make is that being overly nervous about taking tests is wasted energy. If you are confident that you have had and will continue to have a successful academic career then have the same confidence that you will do well on this test.
Don't misunderstand me. I believe that a small amount of nervousness is a good thing. If we didn't have an inkling of nerves life would be somewhat dull. And if my kids expect to take a test and pass with flying colors without proper studying and preparation then they should be nervous.
So the next time you find yourself getting nervous about taking a test, ask yourself if you have done your best to prepare. And know that in 2 years, 5 years, even 20 years this single test won't matter. The only thing that will matter is your cumulative work.