Quiz

I don't think this would be very much work, if you understand question behaviours. (Possibly big if that.)

"the feedback would explain which tests were appropriate for that case and give the test results" - I am pretty sure this information belongs in the General feedback section of the question, so no work to do that. We just need to control when that is displayed.

And, a question beahviour is the right place to control that. Start by looking at the 'adjust_display_options' method of qbehaviour_manualgraded (and hence of the question_behaviour base classe).

In fact, that probably already does what you want. It looks like what you really want here is a 'Check' button on Essay questions, so that students can submit their response, and get the feedback, and then no be able to edit their response further. So, look at how qbehaviour_immediatefeedback works.

At any rate, it should be possible to work out exactly what you want, and make a behaviour that does that, and probably all you need to do is merge the right bits of qbehaviour_immediatefeedback and qbehaviour_manualgraded.

Then there is the question of how do you get Moodle to use that behaviour with your questions. Well, The basic code flow looks like this:

1. Quiz has a setting ->preferredbahaviour, which is what the teacher set in the quiz settings.
2. When each question is started, we need to know what real behaviour to use for that question attempt.
3. So the questoin engine calls $question->make_behaviour($qa, $preferredbehaviour);
4. That method returns an instance of the right behaviour. For may question that will be $preferredbehaviour, but there are exceptions. For example for essay questoins, we always return qbehaviour_manualgraded, no matter what the teacher chose.

Probably what you need to do is create a new question type, which is almost exactly like essay (subclass everything, and override the minimum amount necessary) but which overrides make_behaviour. You could call this question type somthing like "Collect response then give feedback". (Not a great name, try to think of something better.)

Alternatively, just hack qtype_essay_question::make_behaviour to create an instance of your new behaviour when the preferred behaviour is immediatefeedback. I would certainly start by doing that while testing your behaviour.