Payment parameter 'Enable test mode' does not enable 'Test mode result' dropdown when checked

On the Payment settings page -> Worldpay settings fieldset -> Payment parameters fieldset there are settings to control whether test mode is enabled or not, by default this is enabled and the 'Test mode result' dropdown is enabled so that a transaction result can be selected.

If at some point the 'Enable test mode' checkbox has been un-checked and you want to enable test mode again, checking the checkbox does not enable the dropdown until the page is saved.

I've fixed/improved the usuability by adding some jQuery to toggle the dropdown when the checkbox is checked and un-checked.