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/format_string.lua

@@ -1,5 +1,3 @@
-local luaassert = require('luassert')
-
 local M = {}
 
 local SUBTBL = {
@@ -124,7 +122,7 @@ function M.format_luav(v, indent, opts)
   else
     print(type(v))
     -- Not implemented yet
-    luaassert(false)
+    assert(false)
   end
   return ret
 end