Hi Oleg,
The way I deal with your dilemma, if I've understood it correctly, is to use two different Moodle activities: one for learning activities where they can ponder and have have as many tries as they wish, and another for testing whether or not they have a body of knowledge.That is, I use Hot Potatoes for learning activities, and Quiz for testing. Each of these can be used for both learning and testing, but not equally easily.
With Hot Potatoes it's easy to make the scoring invisible until the activity is finished, to give constructive feedback and, if necessary, links to outside resources (that's my way of doing what you suggest: letting students decide whether they need help or not). Presenting Hot Potatoes through QuizPort (Moodle 1.9) also allows the teacher to stipulate pre and post conditions to guide students through exercises in function of their level. (I don't actually use this: I tried and my adult students expressed extreme frustration!)
With Quiz it's easier to make a real test to evaluate a student's level. I use it for placement testing. It's easy to make it clear there's only one correct answer and no second tries. There is also more variety of question types. And you can analyse the results to see if what you think are "difficult" questions really stump more students.
Using the two different activities/modules also makes it clearer to students what the objective is: learning or testing.
Cheers,
Glenys;
PS. Loved your recent "rants" on a completely different subject. It was always perfectly clear to me that they were controlled "actors" rants.