You have hit on most of the best choices, which are to get the biggest EZ-out that will fit into a new pilot hole drilled in the broken off stub, and weld it to the end of an extension to reach the stub. Drill the stub out to the recommended pilot diameter for the EZ-out with a long HSS drill, then drop dry ice into the pilot hole while running hot water through the engine cooling system. You can drive the EZ-out with a hammer drill to get some impact force on it. If this does not work, then you are faced with the possibility of having to break up the cylinder head in place, at least the side of it where the test valve threads in. This requires a lot of very careful drilling down from the top, and if you hit the head retainer bore you are going to have to have it welded up and re-machined. This particular job is never much fun.