Updated test:CV contents shouldn't be available on client after deleting it from selected env

When a system is already registered with an env or CV, the deletion changeset doesn't allow to delete published contents from the selected env. And UI raised an error saying: 
`system is already subscribed to this view, move the system to other env or cv.`

So I updated the test accordingly. But I couldn't catch the exact error as UI throws, because when promotion fails,  `promote-or-delete` fn of `katello/changesets` throws their own defined error.