This parallels question_attempt->minfraction, which allows the
fractional mark to go below zere.
This is needed to allow the certainty-base marking behaviours to work
better.
1/ type/match/tests/walkthrough_test.php - tests are failing randomly, looks like some weird randomisation is going on there - see TODOs
2/ type/multianswer/tests/upgradelibnewqe_test.php contains invalid expected value - see TODO