Demo's are great ways to test performance on my computer
Actually, demos aren't alwayas a good indicator. For example, the Medieval II: Total War demo ran terribly on my system, so bad that I was disheartened that I wouldn't be able to play it. Then I bit the bullet and bought it anyway and it ran great.
is there a way for developers create a small file size, non interactive, program that will demonstrate how the game will run on a computer?
Some games like F.E.A.R. have little performance tests that give a nice cross section of everything you'll encounter in a game to adjust your settings. These tests should be downloadable for free for every game.