Guys you don't need a SIM (their site even says remove it). The only reason you would need a SIM is if you leave the phone at the white "hello"screen after restoring the phone to stock as you cant get the phone past the setup again without the SIM (it wont activate in terms of Apple/itunes). They hook it up to a machine that tests if everything works on the device, and it needs the phone through the intial setup.
Just go through the setup not putting in a pin, touch id, no siri setup, etc and then bring it in. MAKE SURE YOU CHARGE IT TOO the test takes a few minutes, dont be that guy bringing it in with 10% battery.
I brought my SIM but never needed it as I setup my phone as new and went through the initial setup.