Besides competing in a tournament, a belt test is probably the most nerve-wracking part of martial arts.1
Belt tests in my alma mater focus more on the upper ranks: Black belt tests can be 45 to 90 minutes long, but lower rank tests are pretty short: usually about 15 – 25 minutes long.
Why so short, if you’ve been training for months? The idea is that the instructors already know how well their students are doing; the test is really more of a recital than anything else. (But again, at higher ranks the test carries more weight.)
The white belt test is really short. Its main purpose is to determine if anyone is exceptionally talented or experienced. Everyone comes in as a white belt, of course, even if you come in from another school2, but if you hold black belts in other disciplines, chances are that you’ll dispense with some of the beginners training. That’s the main purpose of a white belt test: to determine how far the student will advance. Not if, but how far.
The only way to fail a white belt test is to walk off the mat before it’s over.
My mother recently reminded me that I took one of my AP tests in Latin, which I didn’t remember doing (it was thirty years ago). I don’t remember the idea, or the reasoning that led me to doing it. It makes sense, though, if I remember correctly.
I mean, I really sucked at Latin in high school. And here I was taking my AP test in it. Why? Because no matter how bad I was at Latin, the practice test was still unbelievably easy. Like, my straight C average in Latin in high school translated to an A in the AP test world. As usual, I was my own worst critic. And if I hadn’t taken the practice test, I might never have found out that I could pass the real one.
Who knows how well we’re doing? Don’t doubt yourself TOO much; you’re also probably your own worst critic. Just do the best you can.
And don’t leave the mat until the test is over.
 There are some schools who don’t make an entire production out of a test, but who spring them on you one day unexpectedly, or even just abandon them altogether and present new ranks whenever the instructor deems the students ready. I think I favor a hybrid approach myself, but that’s another story.