My question still stands. How do you evaluate someone's half-finished work? Do you prefer if I finish more features or my already finished code is better quality or better tested?

It's just an unnecessary source of stress during the test.

It would probably be best if someone was available to answer these kinds of questions for the interviewee when their time period starts. Though I think it could also be interesting to have this be part of the assessment -- how does the candidate trade off between scope / quality when they are given a fixed time period?

It is as interesting as evaluating them based on how they indent their code. I can do both but if you don't tell me what you prefer how can I choose?