Your question was whether you could download the AHT. You can't.
You probably could run it from a USB drive, but only by copying the original disc. If the disc is damaged or the optical drive isn't working, I don't see how that would help you.
The disc and drive are perfect.
When you say "only by copying the original disc" do you mean because each disc is specifically tailored for the hardware that it shipped with?
or do you mean I should copy the contents of the entire dsic to USB( rather than extratc the AHT pkg?)
If you want to make a bootable USB device from the disc, use the Restore tab in Disk Utility. Search its built-in help for the term "duplicate" to find details.
Retrieving data ...