Well, I should not be a single point of failure in this. In principle, anyone with the necessary knowledge of how Moodle works should be able to develop this. In practice, since this requires understanding both Moodle Files API and the Moodle question engine (and possibly the PHP unit integration) the number of people who can do this are limited. I don't even know exactly what needs to be done to make this work, and I won't until (if) I try to code it myself.
The question engine is designed so that it was possible for the unit tests to be run entirely in memory. The File API is not like that. It may well be necessarly to do unit tests here that acutally save to the database, but given what PHP unit can do (in comparison to the old Simpletest) that may not really be a problem.
By all means please try to work this out. The uni tests need to simulate what happens to a question in mod/quiz/startattempt.php, attempt.php and processattempt.php. All that code exists (and is explained at http://docs.moodle.org/dev/Using_the_question_engine_from_module). You just need to extract it, make it more abstract, and put it in a unit test.