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/terminal/api_spec.lua

@@ -48,8 +48,8 @@ describe('api', function()
       {3:-- TERMINAL --}                                    |
     ]])
 
-    ok(socket_session1:request("nvim_ui_attach", 42, 6, {rgb=true}))
-    ok(socket_session2:request("nvim_ui_attach", 25, 30, {rgb=true}))
+    ok((socket_session1:request("nvim_ui_attach", 42, 6, {rgb=true})))
+    ok((socket_session2:request("nvim_ui_attach", 25, 30, {rgb=true})))
 
     socket_session1:notify("nvim_input", "\n[socket 1] this is more than 25 columns")
     socket_session2:notify("nvim_input", "\n[socket 2] input")