Instead of having the test runner do the extraction, make can do it for
us. This allows for better parallelization. It also lets the WADs be
extracted separately (and potentially modified) before the tests are
run; I'm planning to reuse this for wadptr so that I can test WADs play
back as intended after being compressed.
This is what is currently generated by Chocolate Doom at Git HEAD
version (e1cc501e1a775490abd). It has not been tested with other
source ports or generated from Vanilla Doom.
From <https://github.com/fragglet/miniwad>
This is complete enough to be able to play back demos from PWADs;
since it all runs headless we don't care about the blank textures.
