vim-patch:9.0.0341: mapset() does not restore <Nop> mapping properly

Problem:    mapset() does not restore <Nop> mapping properly.
Solution:   Use an empty string for <Nop>. (closes vim/vim#11022)
https://github.com/vim/vim/commit/92a3d20682d46359bb50a452b4f831659e799155

Problem:    maparg() does not provide enough information for mapset().
Solution:   Add "lhsraw" and "lhsrawalt" items.  Drop "simplified"
https://github.com/vim/vim/commit/9c65253fe702ea010afec11aa971acd542c35de2

vim-patch:9.0.0127: unused variable

Problem:    Unused variable.
Solution:   Remove the variable. (closes vim/vim#10829)
https://github.com/vim/vim/commit/e95f22f63a1871b91e5508088e5ae4905ce28cd7

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