Stage 1 is all about passing tests - wear whatever makes you feel comfortable. If you're going to worry that slightly shabby dress might affect your chances (it won't AT THIS STAGE) then dress smart. If you feel uncomfortable and can't relax as well in a suit then dress casual.
There is nobody from NATS 'assessing' you at stage 1.
As for the interview stages - I think only a fool would turn up in anything less than shirt and tie.
Good luck all