test: replace busted with local harness

Replace the busted-based Lua test runner with a repo-local harness.

The new harness runs spec files directly under `nvim -ll`, ships its own
reporter and lightweight `luassert` shim, and keeps the helper/preload
flow used by the functional and unit test suites.

Keep the file boundary model shallow and busted-like by restoring `_G`,
`package.loaded`, `package.preload`, `arg`, and the process environment
between files, without carrying extra reset APIs or custom assertion
machinery.

Update the build and test entrypoints to use the new runner, add
black-box coverage for the harness itself, and drop the bundled
busted/luacheck dependency path.

AI-assisted: Codex
(cherry picked from commit 55f9c2136e)

.github/workflows/test.yml

@@ -74,10 +74,6 @@ jobs:
         name: luals
         run: cmake --build build --target luals
 
-      - if: success() || failure() && steps.abort_job.outputs.status == 'success'
-        name: luacheck
-        run: cmake --build build --target lintlua-luacheck
-
       - if: success() || failure() && steps.abort_job.outputs.status == 'success'
         name: lintsh
         run: cmake --build build --target lintsh