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

test/unit/tempfile_spec.lua

@@ -46,7 +46,7 @@ describe('tempfile related functions', function()
 
     itp('generate name of non-existing file', function()
       local file = vim_tempname()
-      assert.truthy(file)
+      assert(file)
       assert.False(lib.os_path_exists(file))
     end)