The only thing that has helped (for me) is talking to my tp about why tests freak me out so much. I used to base my self-esteem on my test scores, which was pretty much a recipe for disaster. Now, I still feel jittery but less panicky. For me, listening to music helps- I get to the testing center early, then hole up somewhere with my headphones and listen to music that reminds me of good times in my life until right before the test to get through the time before it.