Well, obviously you have to do system demonstrations and/or validation of predictions, but the use of CFD-like predictive methodology to conservatively predict ice shapes for the design point conditions is quite common. I believe that all Bombardier's products have used some combination of natural icing testing (priomarily to demonstrate functioning of the AI systems) and artificial ice shape testing to demonstrate handling compliance.
The codes used to generate the artificial shapes all have to be approved by the relevant airworthiness authorities - there are various codes used by differnt companies and in different countries.
The problem with trying to use natural icing to demonstrate handling compliance is that you can almost never get the right flight conditions - you'll either get favourably good conditions, and may 'pass' a test that you should not, or you'll get conditions beyond the accepted worst-case and fail a test that you should pass.
It's also not (just) accretion up to a shed point. There's a time limit for the exposure based on the severity of the conditions and what you're testing ('failure' cases, or unprotected shapes, or 'turn on' shapes) and there's also a size limitation which is based upon 'accepted' shedding geometry (but which ends up being relatively penalising in practice)