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.

src/testdir/test_messages.vim

@@ -360,12 +360,13 @@ func Test_fileinfo_after_echo()
 
     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>")

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4577,
 /**/
     4576,
 /**/