testing/tic-80: don't build with pro enabled

The licence technically allows us to do this (TIC-80 is free software, after all), but I don't think it's right. The BUILD_PRO build option is meant to build the paid version of TIC-80 that's sold to support development and is not intended to be enabled in redistributed builds like this. As such, I think we should appreciate that TIC-80 upstream is keeping the paid version 100% free software rather than going the free core route, and disable this option here out of respect.