patch 8.2.3608: users who type "q:" instead of ":q" are confused

Problem:    Users who type "q:" instead of ":q" are confused.
Solution:   Add an autocmd to give a message that explains this is the
            command-line window. (Egor Zvorykin, closes #9146)
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index 87ae6fa..295e59d 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -1255,6 +1255,7 @@
   CheckScreendump
 
   let lines =<< trim [SCRIPT]
+    augroup vimHints | au! | augroup END
     call setline(1, range(30))
     2split
   [SCRIPT]