Add example of multipass being used instead of a grader

[Tagl]

One thing we believed the new format wouldn't support, that the old graders did, was something like:
"You must solve at least 60% of the test cases"
This can be implemented with multipass though, as each run will look like a separate test case from the perspective of the submission.
It would be good to add an example problem that does something similar to this.