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/vterm_spec.lua

@@ -275,6 +275,13 @@ local function push(input, vt)
   vterm.vterm_input_write(vt, input, string.len(input))
 end
 
+-- vterm_input_write() can synchronously invoke the Lua callbacks installed
+-- above. LuaJIT must not JIT-compile an FFI call that re-enters Lua, or the
+-- test can panic with "bad callback".
+if jit then
+  jit.off(push, true)
+end
+
 local function expect(expected)
   local actual = read_rm()
   t.eq(expected .. '\n', actual)