You could try the giant demon who wanders around the plateau on the far left side of Nagrand:
Reth'hedron the Subduer. He's level 73 and (as far as I know) deals melee damage with some AOE spell damage mixed in. I've heard of others suggesting him for level-73 mob DPS tests, so you could try for a tanking test as well.
As for taking enough damage, if you aren't taking enough damage try un-equipping some threat-neutral pieces. (Using mitigation only necklace? Take it off to take more damage.) If you are sitting right at uncrushable, then taking off gear turns pure avoidance into hits, not into blocks. (The only potential problem here would be when redoubt starts ticking and puts you back at uncrushable, giving you more blocks and therefore more threat. However, if you want to be that precise, try respeccing out of redoubt for the test.)