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. Split the question_attempt tests into one class per file.
2. Imporve the API to give tests more control, and to test more of the
important code. Some of this is not used here, but it is about to be.
Tim Hunt