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)

test/functional/preload.lua

@@ -1,6 +1,6 @@
--- Modules loaded here will NOT be cleared and reloaded by Busted.
--- Busted started doing this to help provide more isolation.  See issue #62
--- for more information about this.
+-- Modules loaded here will not be cleared and reloaded by the local harness.
+-- Keeping these preloaded preserves cross-file setup while still resetting
+-- non-helper modules between files.
 local t = require('test.testutil')
 require('test.functional.testnvim')()
 require('test.functional.ui.screen')