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.
diff --git a/src/testdir/test_messages.vim b/src/testdir/test_messages.vim
index d3c80bf..32689b0 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -360,12 +360,13 @@
 
     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"
   END
 
   call writefile(content, 'Xtest_fileinfo_after_echo')
   let buf = RunVimInTerminal('-S Xtest_fileinfo_after_echo', #{rows: 6})
+  call term_sendkeys(buf, ":set updatetime=50\<CR>")
+  call term_sendkeys(buf, "0$")
   call VerifyScreenDump(buf, 'Test_fileinfo_after_echo', {})
 
   call term_sendkeys(buf, ":q\<CR>")