Add tests and CI

[UnrelatedString]

Description of the change

Added foreign definitions for the test module, machinery for running tests, and a ci.yml that runs them automatically. Found a likely compiler bug and had to comment out one of the test suites, and now one of the tests fails on CI but passes locally for me, but failing tests are better than no tests!

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Checklist:

• Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation
• Added a test for the contribution (if applicable)

[Unisay]

Thanks @UnrelatedString — adding a runnable test suite for the prelude is exactly right, and the testNumberShow cases are useful. I'm closing this PR rather than merging it for two reasons: (1) it has drifted out of date and now conflicts with master (the CI here predates the move to a nix-based workflow, and several FFI files have since changed); (2) test/Test/Main.lua wouldn't pass on the Lua 5.1 target as written — for case in cases do isn't a valid iterator, case[0] is nil (Lua is 1-based), and cases/input/expected/actual leak as globals. I'll bring the test suite back in a follow-up on top of the canonical nix CI, with the Lua FFI corrected and your number-show cases credited. Sorry for the long delay on this.