startup: always wait for UI with --embed, unless --headless also is supplied

test/functional/plugin/helpers.lua

@@ -3,7 +3,6 @@ local paths = require('test.config.paths')
 local helpers = require('test.functional.helpers')(nil)
 local spawn, set_session, nvim_prog, merge_args =
   helpers.spawn, helpers.set_session, helpers.nvim_prog, helpers.merge_args
-local request = helpers.request
 
 local additional_cmd = ''
 
@@ -14,7 +13,7 @@ local function nvim_argv(shada_file)
                      '--cmd', 'set shortmess+=I background=light noswapfile belloff= noshowcmd noruler',
                      '--cmd', 'let &runtimepath=' .. rtp_value,
                      '--cmd', additional_cmd,
-                     '--embed'}
+                     '--embed', '--headless'}
   if helpers.prepend_argv then
     return merge_args(helpers.prepend_argv, nvim_args)
   else
@@ -30,7 +29,6 @@ local function reset(...)
   end
   session = spawn(nvim_argv(...))
   set_session(session)
-  request('nvim_eval', "0")
 end
 
 local function set_additional_cmd(s)