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.

Discovered using --shuffle argument of busted.

Extend the capabilities of is_os to detect more platforms such as
freebsd and openbsd. Also remove `iswin()` helper function as it can be
replaced by `is_os("win")`.

Problem:    Vim9: error for not using string doesn't mention argument.
Solution:   Add argument number.
https://github.com/vim/vim/commit/f28f2ac425600b88da0bdcc12a82cd620f575681

exepath and stdpath should respect shellslash and return path with
proper file separator.
Closes #13787

Cherry-pick check_for_nonempty_string() from patch vim-8.2.2133 and
apply it on the bases of https://github.com/neovim/neovim/pull/13489

https://github.com/vim/vim/commit/2a9d5d386bea8455b37c1accebc45683ec51d6fb

Problem:
Subdirectories like "visual", "insert", "normal" encourage people to
separate *related* tests for no good reason.  Typically the _mode_ is
not the relevant topic of a test (and when it is, _then_ create
an appropriate describe() or it()).

Solution:
- Delete the various `test/functional/<mode>/` subdirectories, move
  their tests to more meaningful topics.
- Rename `…/normal/` to `…/editor/`.
  - Move or merge `…/visual/*` and `…/insert/*` tests into here where
    appropriate.
- Rename `…/eval/` to `…/vimscript/`.
  - Move `…/viml/*` into here also.

* test(reorg): insert/* => editor/mode_insert_spec.lua
* test(reorg): cmdline/* => editor/mode_cmdline_spec.lua
* test(reorg): eval core tests => eval_spec.lua