MDL-39980 question engine: Attempt on last loses response files

When doing Each attempt builds on last, we need to copy any response
files into a draft file area, and then re-save them.

While writing the unit test for this, I had to deal with a todo in the
question engine so that questions with files in the response could be
unit-tested.

I also found an fixed a bug with qtype_essay_question::is_same_response
and fixed some notices in the existing essay/manual graded unit tests.