patch 9.0.0306: buffer write message is two lines in message popup window

Problem:    Buffer write message is two lines in message popup window.
Solution:   Overwrite message if "msg_scroll" is off.
diff --git a/src/testdir/test_messages.vim b/src/testdir/test_messages.vim
index 7c71edc..f714478 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -494,13 +494,17 @@
   call VerifyScreenDump(buf, 'Test_cmdheight_zero_3', {})
 
   " Message for CTRL-C is in the popup window
-  "call term_sendkeys(buf, ":call popup_clear()\<CR>")
   call term_sendkeys(buf, "\<C-C>")
   call VerifyScreenDump(buf, 'Test_cmdheight_zero_4', {})
 
+  " file write message is one line
+  call term_sendkeys(buf, ":w XsomeText\<CR>")
+  call VerifyScreenDump(buf, 'Test_cmdheight_zero_5', {})
+
   " clean up
   call StopVimInTerminal(buf)
   call delete('XtestCmdheight')
+  call delete('XsomeText')
 endfunc
 
 func Test_cmdheight_zero_shell()