Add a test that demonstrates problems with custom menu keybindings

The test shows two problems: a <down> keybinding is not removed from a menu item
(the 'y' binding is removed though, which is correct), and keybindings such as
'j' and 'H' don't work. We will fix both of these separately in the following
commits.

pkg/integration/tests/test_list.go

@@ -170,6 +170,7 @@ var tests = []*components.IntegrationTest{
 	custom_commands.BasicCommand,
 	custom_commands.CheckForConflicts,
 	custom_commands.CustomCommandsSubmenu,
+	custom_commands.CustomCommandsSubmenuWithSpecialKeybindings,
 	custom_commands.FormPrompts,
 	custom_commands.GlobalContext,
 	custom_commands.MenuFromCommand,