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`.
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.
Update vim_diff.txt with :lmap differences, update documentation on
'keymap', and add tests.
The tests added are to demonstrate the behaviour specified in the
documentation of :loadkeymap.
This means that the major way that :lmap mappings are applied works as
one would expect with macros.
This also means that having a translation with 'keymap' does not
preclude using mappings in insert mode with :imap.
There is some behaviour that we keep with the recent changes, and some
behaviour that we change.
Instetad of having one failing test covering all behaviour, we split
the test into two.
