Sadly the tray loader pre-dates any sort of Apple Hardware Test CDs...IIRC they came in with the Summer 2000 imacs.

I've noticed that in the months before it died, its been taking longer for the display to warm up, despite the fact that the system has always powered on straight away. (in fact the last time I used it before it died, by the time the display had warmed up enough for it to display an image, the machine was fully booted and ready to roll)

I've got several ATX PSUs kicking around, maybe it might be time for me to at least try doing an ATX hack...