Unfortunately, the AHT disk is not available other than with the purchase of a Mac or via eBay, but then only if you can be sure the disk is for the exact same Mac model you've got. No downloads either, AFAIK.
The test account showing the same problem suggests that it is system-wide and may require reinstallation of the OS to fix (if you haven't already tried that). Of course, the issue may very well be hardware related and in fact that's what it looks like, in which case reinstallation won't help much. OTOH, there's little to lose at this point.
