Problem: A child root=false masks a parent root=true during merging.

Solution: Return the search boundary separately from string properties.
Exclude root from property callbacks and b:editorconfig.

AI-assisted

Problem:
During insert-mode / replace-mode, `autowrite` may trigger. If it does, the
cursor position can shift due to the automatic removal of trailing spaces on the
current line. When I resume typing, the space between the last word and the new
word is suddenly gone.

Solution:
Disable the "remove trailing spaces" handler during Insert (or a similar) mode.
Autosave logic is not affected.

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: A custom property containing a pair of square brackets will be
treated as a section.
Solution: Change the logic parsing a section, remove the first match
regex `%b[]`.

Signed-off-by: fortime <palfortime@gmail.com>

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.