When I took mine a few days ago, I wore really comfy gauchos (they feel like pj's!), flip flops, and a t-shirt. I brought a zip up sweater because I get cold at the drop of the hat and each testing center is different...when I took mine the first time, the place was freezing!!! I was so glad I had a sweater. But honestly, I did not want to be all dressed up just to smile for the camera and take the hardest test of my life! They won't turn you away if you're dressed super casual.
But they do give great tips if you show up in your birthday suit!