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.

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

Problem:    Cannot get all information about a quickfix list.
Solution:   Add an optional argument to get/set loc/qf list(). (Yegappan
            Lakshmanan)

https://github.com/vim/vim/commit/d823fa910cca43fec3c31c030ee908a14c272640

Problem:    Arguments of setqflist() are not checked properly.
Solution:   Add better checks, add a test. (Nikolai Pavlov, Hirohito Higashi,
            closes vim/vim#661)

https://github.com/vim/vim/commit/d106e5ba7f10f0d2a14eaefe5d78405044416cb9

It is otherwise impossible to determine which test failed sanitizer/valgrind
check. test/functional/helpers.lua module return was changed so that tests which
do not provide after_each function to get new check will automatically fail.

All syntastic users experienced this problem:

  E685: Internal error: get_tv_string_buf()

It's reproducable with:

  :call setloclist(0, [''])

So, not given optional arguments to setloclist() lead to some fields not
inizilied and the code took the wrong branches.