Problem:
When disabling, the loader list is traversed forwards while entries are
removed. As a result, some entries are skipped and remain active. Thus,
disabling `vim.loader` restores `_G.loadfile`, but does not fully
restore the original package loader chain.

Solution:
Traverse `package.loaders` in reverse when removing cached loaders.
Capture the original `package.loaders` list in the test, and assert that
the list is restored properly.

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`.

Problem:
`vim.loader.disable` does not conform to `:help dev-name-common` and
`:help dev-patterns`.

Solution:
- Add `enable` parameter to `vim.loader.enable`
- Remove `vim.loader.disable`
- Note the change in `:help news-breaking-dev` (HEAD changes).
  - This is not a breaking change (except to "HEAD") because
    `vim.loader` is marked "experimental".

previous: 26765e8461c1ba1e9a351632212cf89900221781

Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.

Closes https://github.com/neovim/neovim/issues/27004.

Work on https://github.com/neovim/neovim/issues/27004.