Test: add separate test suite to demonstrate bugs

Creates directory counterexamples, with own dune config file and an initial counterexample demonstrating a bug.

The nominal purpose of this directory is to provide a staging-ground to hold source-files that individually expose existing bugs in the library, to more easily identify what changes are required to fix those bugs, and to know immediately when they have been fixed.

Can be invoked from root with dune run counterexamples