I hear it all the time; "I get so nervous before a test." Usually followed by questions like, "Do you think I'm ready to test?" I suppose that many years ago, martial arts were taught very different than today. For instance, the practice of changing belts from white to increasingly darker colors, ending in black is relatively new. Originally a student received a white belt and it simply became darker with age and wear.
I trained for years with both of my children. Eventually my daughter went off to college, but before that I used to get incredibly nervous before a test; not so much for myself but for them. As it turns out, their young brains were able to grasp and retain the material much better than mine. Ah, youth is wasted on the young.
Today, we use testing as a measure of progress and preparedness to learn new material. A White Belt practitioner is tested to demonstrate mastery (a white belt master?) of very basic material and their readiness to learn Yellow Belt material. In a well-run school a student would never be placed in a situation where they could fail the test (note exception below). The instructor should not allow a student to test if they are not prepared.
In a very real sense, therefore, if you are notified that you are going to be tested, then it likely means your instructor has already determined you are ready and you will likely pass. This is assuming, of course, that you are attending a well-run school with an experienced Instructor. Some schools will tell you that it takes 'X' amount of time to become a Black Belt. In my discipline it is approximately four years. Very few people do it faster.
We can give a time frame like this, even though we know everybody learns at a difference pace because we have a highly structured program where the material has been carefully distributed across the various belts so that the student never has much to learn at any one point. Still, if the student does not practice or attend class, the three months between yellow and orange can go by too fast. Sometimes, we have to postpone a test until the practitioner has demonstrated fluency with the material.
Again, if the instructor asks you to test, or puts your name on the test list, then it likely means he or she has observed you and determined you are ready. But be careful with this knowledge. Just because you are ready to test does not mean you cannot fail. Arrogance can lead to a lack of concentration, and during a test, a simple, brief loss of concentration can be anywhere from embarrassing to dangerous.
In the advanced ranks it is absolutely possible to fail a test. I've seen it happen to black belts and even masters. As you progress, more is expected of you. I once saw a test for Kyosa, or certified instructor, where the Grand Master sat everyone down after only five minutes of testing and said, "You all flunk." Turns out they, as a group, failed to demonstrate confidence in their verbal responses to various instructions.
Lastly, as you progress through the ranks, your nerves should subside. Not only will you become more familiar with the testing process, but your martial arts training should enable you to gain more control over your feelings.