Add option to *only* be able to see the results of a test *after* its expiry date
In some cases, it might be interesting to have an additional special "feedback" option: "Wait for exercise's expiry date to show the results".
This would make it impossible for users to see their results to the test before the expiry date (limits cheating).
This option shouldn't be mixed with the normal feedback options, because it extends all of them (it's not "one more mode").
This was suggested by UTP.