Use set difference (ANSWERED - JUDGED) to determine whether to advance

This is a bug fix for the bug described and tested in the previous
commit. Previously we were simply comparing the count of players who
had judged with the count of those who had answered.

Here, instead, we compute an actual set difference and only
auto-advance from the judging phase when every player who answered has
also completed judging.

This fix causes the test added in the previous comit to start passing,
so now the entire test suite is passing once again.