vim-patch:8.2.4577: message test is flaky

Problem:    Message test is flaky. (Elimar Riesebieter)
Solution:   Trigger the autocommand event only after startup is finished.
9323ca51c2

test/functional/legacy/messages_spec.lua

@@ -54,9 +54,10 @@ describe('messages', function()
 
       hide buffer a.txt
 
-      set updatetime=1
-      autocmd CursorHold * b b.txt | w | echo "'b' written"
+      autocmd CursorHold * buf b.txt | w | echo "'b' written"
     ]])
+    command('set updatetime=50')
+    feed('0$')
     screen:expect([[
       ^hi                                      |
       {1:~                                       }|