Organize tests into subdirectories and use a new makefile that scans
.sby files and allows selecting tests by mode, engine, solver and/or
subdirectory. Automatically skips tests that use engines/solvers that
are not found in the PATH.
See `cd tests; make help` for a description of supported make targets.
This also changes the test Makefile to run `.check.py` files after
running the corresponding `.sby` file to allow more precise testing of
the keep going feature.
