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/functional/testnvim.lua

@@ -1,6 +1,7 @@
 local uv = vim.uv
 local t = require('test.testutil')
-local busted = require('busted')
+---@type test.harness
+local harness = require('test.harness')
 
 local Session = require('test.client.session')
 local uv_stream = require('test.client.uv_stream')
@@ -528,7 +529,7 @@ function M.new_session(keep, ...)
   return new_session
 end
 
-busted.subscribe({ 'suite', 'end' }, function()
+harness.on_suite_end(function()
   M.check_close(true)
   local timed_out = false
   local timer = assert(vim.uv.new_timer())