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

Problem:    Incorrect error messages for functions that now take a Blob
            argument.
Solution:   Adjust the error messages. (Dominique Pelle, closes vim/vim#3846)
https://github.com/vim/vim/commit/0d17f0d1c09fa6db306336695ba646c21ea24909

closes #6035
closes #9250

closes #5442
closes #4142
ref #6618
ref #4376
ref #7844
ref #2958
ref #4338

And similarly nvim_command_output test