Slight overall improvements to the QuickCheck module:

  - Fixed bug that caused some QuickCheck tests to throw an exception because not a complete substitution was taking place.
  - Uncommented the legendary 16th bug that causes an endless loop when Z3 is used. It doesn’t cause an endless loop with QuickCheck, but QuickCheck does give the wrong answer :’(.

All in all, the very naive testing module correctly tests 13/16 tests at the moment (most of the time at least; it’s random).