I think it depends on what you mean by "user testing". Rather than focus groups and asking users to design to app for you (which would be bad) I assume what is being done would be to provide a few sample interfaces and record users doing various tasks. You can also track eye movements and see which parts of the interface they concentrate on most.

If they take 10 minutes to figure out one interface and 10 seconds to figure out another one for example it's quite obvious which one is more intuitive.