This is a technique used by athletes, entertainers, and anyone preparing for a major event in their life.
Imagine yourself in the test, prior to actually being there.
Imagine the inside of the test centre (or even better, find a picture of it). Think what you’ll feel like when you sit down. Imagine what it’ll be like when you see the questions.
Making things “seem” more familiar will help relax you and aid your concentration.
This “mental imagery” may seem strange at first, but with practice you’ll see how it benefits you.