Feature wish: handle several different graders for the same exercise

[erikmd]

• As a teacher using learn-ocaml,
• I would like to easily handle a set of (at least) 2 different graders (test.ml) for a given subject (descr.md),
• In order to allow students to exercise themselves with test_1.ml, then use a more thorough grader implementation test_2.ml to give students a final grade for the assignment.

This appears to be an important feature when we focus on graded assignments, and currently it is only done manually by having two different exercises {exo-v1, exo-v2}/{descr.md, test.ml, solution.ml, etc.} and typically checking manually that exo-v2/solution.ml gets a 100% grade with exo-v1/test.ml.

@YoanwM recently started working on this feature, just opening an issue as some heads-up / to track progress on this.