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/lua/net_spec.lua

@@ -199,7 +199,7 @@ describe('vim.net.request', function()
       return result
     ]])
 
-    assert.is_table(headers)
+    t.eq('table', type(headers), 'Expected headers to be a table')
     -- httpbingo.org/request returns each header as a list in the returned value
     t.eq(headers.Authorization[1], 'Bearer test-token', 'Expected Authorization header')
     t.eq(headers['X-Custom-Header'][1], 'custom-value', 'Expected X-Custom-Header')