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/testtest_spec.lua

@@ -1,5 +1,4 @@
 local t = require('test.unit.testutil')
-local assert = require('luassert')
 
 local itp = t.gen_itp(it)
 
@@ -11,7 +10,7 @@ if os.getenv('NVIM_TEST_RUN_TESTTEST') ~= '1' then
 end
 describe('test code', function()
   itp('does not hang when working with lengthy errors', function()
-    assert.just_fail(('x'):rep(65536))
+    error(('x'):rep(65536), 0)
   end)
   itp('shows trace after exiting abnormally', function()
     sc.exit(0)