I didnt wear a suit for my phase 2, nobody that was in my phase 2 group did. They want you to be comfortable and relaxed at the test so you can perform at your best. However take a suit with you because if you pass phase 2 your interview will take place the next day.
I had my interview in july and it wasn't as bad as i thought it would be, they make to feel welcome and relaxed. Its good to keep in mind that they make their final decision on the whole selection process from phase 1 to the final interview, so it doesnt rest completley on a good interview.(but it helps)!!
Good luck with phase 2!