patch 8.1.2416: loading menus sets v:errmsg

Problem:    Loading menus sets v:errmsg.
Solution:   Avoid setting v:errmsg and add a test for that. (Jason Franklin)
diff --git a/src/testdir/test_menu.vim b/src/testdir/test_menu.vim
index efbb982..0d6b78e 100644
--- a/src/testdir/test_menu.vim
+++ b/src/testdir/test_menu.vim
@@ -10,7 +10,13 @@
     call assert_report('error while loading menus: ' . v:exception)
   endtry
   call assert_match('browse confirm w', execute(':menu File.Save'))
+
+  let v:errmsg = ''
+  doautocmd LoadBufferMenu VimEnter
+  call assert_equal('', v:errmsg)
+
   source $VIMRUNTIME/delmenu.vim
+  call assert_equal('', v:errmsg)
 endfunc
 
 func Test_translate_menu()