As @harple said, it’s stage fright. People have a number of options for dealing with stage fright. You have your chemical solutions and your mental solutions. There are anti-anxiety drugs that you can take to calm you.
However most people prefer the mental techniques. One, as @harple said, is to talk some realism to yourself. Everyone takes this test and almost all of them pass it. The test giver wants you to succeed. He or she is not really trying to make you fail. They just want to make sure you are safe.
There are breathing techniques that can help. Start taking long slow breaths—slowly in and out. Count the seconds on inbreath and out breath and try to make them equal and try to make them longer. This allows you to focus on your task—driving, without having any mental time left for anxiety.
You can also practice giving up. Some people do this with prayer—giving the future up to God. Others just realize that it’s too late to do anything else now. You can’t prepare any more. All you can do is what you do. Whatever you do is what you do. This takes the pressure off because you realize you are no longer in control. Circumstance (or God) are in control. Your job is to merely drive the car. The universe will tell you if you pass or not.
This is also called giving up your attachment to the results. You are taking the test for the sake of taking the test and care not for the result. This is a good practice for anything you do. Focus on the doing of it, and let the rest take care of itself. If you enjoy driving, then just enjoy it, without worrying about being judged. For you, the driving is important. Your job is not to judge. So you don’t even have to think about it.
Finally, there’s nothing like preparation. It gives you confidence. If you feel like you know what you are doing, you just do it without worrying about what anyone thinks.
Good luck.