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

Problem:
- Unix sockets are created in random /tmp dirs.
  - /tmp is messy, unclear when OSes actually clear it.
  - The generated paths are very ugly. This adds friction to reasoning
    about which paths belong to which Nvim instances.
- No way to provide a human-friendly way to identify Nvim instances in
  logs or server addresses.

Solution:
- Store unix sockets in stdpath('state')
- Allow --listen "name" and serverstart("name") to given a name (which
  is appended to a generated path).

TODO:
- is stdpath(state) the right place?

PROBLEM
------------------------------------------------------------------------
$NVIM_LISTEN_ADDRESS has conflicting purposes as both a parameter ("the
current process should listen on this address") and a descriptor ("the
current process is a child of this address").

This contradiction means the presence of NVIM_LISTEN_ADDRESS is
ambiguous, so child Nvim always tries to listen on its _parent's_
socket. This is the cause of lots of  "Failed to start server" spam in
our test/CI logs:

    WARN  2022-04-30… server_start:154: Failed to start server: address already in use: \\.\pipe\nvim-4480-0
    WARN  2022-04-30… server_start:154: Failed to start server: address already in use: \\.\pipe\nvim-2168-0

SOLUTION
------------------------------------------------------------------------

1. Set $NVIM to the parent v:servername, *only* in child processes.
   - Now the correct way to detect a "parent" Nvim is to check for $NVIM.
2. Do NOT set $NVIM_LISTEN_ADDRESS in child processes.
3. On startup if $NVIM_LISTEN_ADDRESS exists, unset it immediately after
   server init.
4. Open a channel to parent automatically, expose it as v:parent.

Fixes #3118
Fixes #6764
Fixes #9336
Ref https://github.com/neovim/neovim/pull/8247#issuecomment-380275696
Ref #8696

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