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

Make sure that oldwin is not invalid after splitting

Revisit this when porting vim patch v8.1.0892 and related quickfix patches.

The commit summary maybe does not make sense, but calling a function
that does not wait on anything `wait()` makes even less sense.

Problem:    No error when using wrong arguments for setqflist() or
            setloclist().
Solution:   Check for the error.
https://github.com/vim/vim/commit/be7a50c22f63478a6e64fe6b932a847830191b95

Problem:    Extending sign and foldcolumn below the text is confusing.
Solution:   Let the sign and foldcolumn stop at the last text line, just like
            the line number column.  Also stop the command line window leader.
            (Christian Brabandt)

https://github.com/vim/vim/commit/8ee4c01b8c79a29065c1af05e5d9c0721069765f

Closes https://github.com/neovim/neovim/issues/9613