patch 9.0.0351: message window may obscure the command line

Problem:    Message window may obscure the command line.
Solution:   Reduce the maximum height of the message window.
diff --git a/src/testdir/test_messages.vim b/src/testdir/test_messages.vim
index 6bdaedf..10af69c 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -396,6 +396,11 @@
         echowindow a:arg
       endfunc
       echowindow 'first line'
+      func ManyMessages()
+        for n in range(20)
+          echowindow 'line' n
+        endfor
+      endfunc
   END
   call writefile(lines, 'XtestEchowindow')
   let buf = RunVimInTerminal('-S XtestEchowindow', #{rows: 8})
@@ -407,6 +412,9 @@
   call term_sendkeys(buf, ":call popup_clear()\<CR>")
   call VerifyScreenDump(buf, 'Test_echowindow_3', {})
 
+  call term_sendkeys(buf, ":call ManyMessages()\<CR>")
+  call VerifyScreenDump(buf, 'Test_echowindow_4', {})
+
   " clean up
   call StopVimInTerminal(buf)
   call delete('XtestEchowindow')