Problem:
- Since c57f6b28d71d #8519, sockets are created in ~/.local/… but XDG
  spec says: "XDG_RUNTIME_DIR: Must be on the local filesystem", which
  implies that XDG_STATE_DIR is potentially non-local.
- Not easy to inspect Nvim-created temp files (for debugging etc).

Solution:
- Store sockets in stdpath('run') ($XDG_RUNTIME_DIR).
- Establish "/tmp/nvim.user/" as the tempdir root shared by all Nvims.
- Make ok() actually useful.
- Introduce assert_nolog().

closes #3517
closes #17093

Expand mappings first by calling `vpeekc()` directly.

Problem:    Newly added features can escape the sandbox.
Solution:   Add checks for restricted and secure. (Yasuhiro Matsumoto)
https://github.com/vim/vim/commit/3849992b16011e36a5cb5be4b127f843389b96fd

timer_start is missing check_secure.

The timer callback can, for example, call a function defined from outside the sandbox that does
stuff that would be disallowed from inside the sandbox. This is usually not allowed.

Cherry-pick eval.txt change from:
https://github.com/vim/vim/commit/68e6560b84f196c82e27a72669684d5506a3a837

Required for v8.1.2013.

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