So that's how it's done...everyone is sitting at the restaurant, waiting around for the bat phone to ring....'Hey we got reports of icing in Colorado...whoopee....we get to go flying today!"
In-flight testing makes sense because you need to see how the plane flies under those conditions, but I think the wind tunnel would give a much better indication of where the build ups occur, and the ability to really load up the plane safely.
But hey, if that's how it's done...waiting around for the phone call...what a great job...you could stretch that gig out for YEARS.