feat: stdpath('run'), /tmp/nvim.user/ #18993

Problem:
- Since c57f6b28d7 #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

test/functional/vimscript/timer_spec.lua

@@ -96,7 +96,7 @@ describe('timers', function()
     nvim_async("command", "let g:val = 0 | let g:c = getchar()")
     retry(nil, nil, function()
       local val = eval("g:val")
-      ok(val >= 2, "expected >= 2, got: "..tostring(val))
+      ok(val >= 2, '>= 2', tostring(val))
       eq(0, eval("getchar(1)"))
     end)
     feed("c")