test(ui/{cmdline,message}2_spec): reduce flakiness/runtime #34875

Problem:  Storing the configured 'cmdheight' value is scheduled and
          may happen after cmdline2_spec already entered block_mode.
          Excessive wait time for expected screen state due to delayed
          ruler after an error message.
Solution: Only schedule storing the user configured 'cmdheight' if
          v:vim_did_enter is unset. Use regular message instead of error.

runtime/lua/vim/_extui.lua

@@ -104,9 +104,11 @@ function M.enable(opts)
     ext.cmdheight = value
   end
 
-  vim.schedule(function()
-    check_cmdheight(vim.o.cmdheight)
-  end)
+  if vim.v.vim_did_enter == 0 then
+    vim.schedule(function()
+      check_cmdheight(vim.o.cmdheight)
+    end)
+  end
 
   api.nvim_create_autocmd('OptionSet', {
     group = ext.augroup,