Utilize negative tests in QCheck

[jmid]

This is a WIP PR to try out the negative testing primitive Test.make_neg from c-cube/qcheck#244.
It is intended to fix #29

Overall

• this allows negative tests to be marked as such with neg_agree_test_par directly in the source code
• the clunky check_error_count can be eliminated entirely
• magical constants denoting 1, 2, and 4 errors in dune files are eliminated, and
• dune files are generally simplified

As an added bonus, it opens up for switching to an alcotest runner as suggested in #15.

I would therefore argue that this is a clear win! \o/

[jmid]

Rebased to latest main

[jmid]

Test.make_neg is now merged into QCheck master, so this can be picked up again.

[jmid]

Rebased after merging #90

[jmid]

Rebased on latest main, having QCheck 0.19 with Test.make_neg as a dependency.

[jmid]

I just annotated the new DSL-ported tests with suitable neg_ suffixes.
Planning to merge this assuming the CI doesn't complain too much.

[jmid]

(Hopefully) last rebasing on main