Problem:    Undo misbehaves when writing from an insert mode mapping.
Solution:   Sync undo when writing. (closes vim/vim#11674)

https://github.com/vim/vim/commit/3f8f82772313af9f2417b06651f30988b63e1c96

Co-authored-by: Bram Moolenaar <Bram@vim.org>

Allows using `undo!` to undo changes and remove them from the undo-tree. Can only be used for moving backwards in the same undo branch.

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