Yeah, probably a wardrobe and not a makeup test. (And I think they sometimes do shoot tests with doubles who are the same size as the actors before they get down to final fittings.) I see inspiration from Leia's Hoth and Endor commando outfits.
There might even be a good reason for her to blend in with the troops in the plot.
We'll probably see her looking more regal at some point in the film.