Since late December, San Diego autonomous driving outfit TuSimple has run seven semi-truck trips between Tucson and Phoenix without a driver in the vehicle. These driver-out demonstrations aim to prove that TuSimple’s artificial intelligence autonomous driving technology works repeatedly along a real freight hauling route. And this spring, the recently public company hopes to add another layer of proof by delivering actual freight for Union Pacific — one of the nation’s largest railroad shippers. While not groundbreaking by itself, the deal with Union Pacific announced Wednesday aims to provid…