move testing system to Shake

[mwotton]

The testing scaffold is a bit ramshackle: we repeat logic to set up schemas etc in both shell and haskell with runSession.
Would be better to port the horrible Make rules to Shake and use them in both places.

This only refers to the tests, Make is perfectly adequate for generating the actual squealgen script, and I don't want to have to require a working haskell system just to install a script.