The Apple TV has a screen saver that consists of different slides scrolling upwards on the screen. They don't cover the whole screen and the screen also "flips" horizontally every so often. The Apple TV is only 720p though and I would be breaking in the Panasonic 60ST50. Does that resolution matter for break-in?
When I get my UT50, I'm just planning on looping 16:9 content for 100 hours or so with contrast turned down a bit. I'll probably use BBC Planet Earth or Life episodes as they have a mix of colour and are not dominated by dark scenes (so the panel should be hit relatively evenly).
You can't beat the slides. Using any other connected device puts you at the risk of them freezing, shutting off, etc. Slides on a flash drive/SD card are your best bet and you'll know you're getting the best possible break-in with pretty much zero risk.