These guys were literally inventing the wheel, so there's probably tons of test footage we've never seen. Several of the explosions seen in the Making of Star Wars appear to be test shots.
I find the revelation about the LED engine lights on the TIE's really neat, as the original Kenner toys had them to simulate laser fire. LED's are so ubiquitous now, it's hard to recall how cool they were having them on a kid's toy in the 70's.