Problem:
The magic globals `it`, `describe`, etc., are more trouble than they are
worth.

- Hooking into `after_each` requires `getfenv()` hacks.
- They confuse luals/emmylua, because the top-level `.luarc.json` isn't
  merged with `test/.luarc.json` (apparently a luals limitation?)
- They totally defeat discoverability because the user just has to
  "know" about the various magic symbols.

So they harm DX, which means they serve no purpose at all.

Solution:
- Expose the test API from `testutil`, so tests can call `t.it()`,
  `t.describe()`, etc., in the conventional way.
- Drop `getfenv()` hacks.
- Drop the `setfenv()` injection in `load_chunk`.
- Drop `test/_meta.lua`.

This is a better way to prevent parallel tests from interfering with
each other, as there are many ways files can be created and deleted in
tests, so enforcing different file names is hard.

Using $TMPDIR can also work in most cases, but 'backipskip' etc. have
special defaults for $TMPDIR.

Symlink runtime/, src/, test/ and README.md to Xtest_xdg dir to make
tests more convenient (and symlinking test/ is required for busted).

Also, use README.md instead of test/README.md in the Ex mode inccommand
test, as test/README.md no longer contains 'N' char.

Closes #33862

Problem:
- cannot run ftplugin_spec.lua by itself
- test leaves foo/ dir

Solution:
- fix setup and teardown

Problem:
Current `'includeexpr'` in runtime/ftplugin/lua.vim doesn't work with Nvim Lua.

Solution:
Provide an improved 'includeexpr' for Lua in "ftplugin/lua.lua".

Closes: https://github.com/neovim/neovim/issues/32490