They take your ID picture, so make sure you look put together and comb your hair haha. Unless you don't really care about that sort of thing. I would say a polo and khakis would be fine, since basically all you do is fill out paperwork.
As a seasoned manager of 30 years, ALWAYS over dress vs. under dress. Even if sitting in a room by yourself for 8 hours, trust me on this... I have interviewed, hired, and on boarded thousands of people in my career.