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/api/server_requests_spec.lua

@@ -170,7 +170,7 @@ describe('server -> client', function()
         if method == "notification" then
           eq('done!', eval('rpcrequest('..cid..', "nested")'))
         elseif method == "nested_done" then
-          ok(false, 'this should never have been sent')
+          ok(false, 'never sent', 'sent')
         end
       end