How to /testforblock in the overworld from the nether
Sorry, but it's not possible. You would have to have both the Nether and Overworld loaded at the same time.
I tested it by using /execute to an armor stand in the Nether from the Overworld. So the problem is the Nether not being loaded when you're in the Overworld.
This is trivial in Minecraft 1.13:
/execute in overworld if block x y z minecraft:stone_button
(/testforblock no longer exists; it was merged into /execute)