Resolves #127: Prevent duplicate report outcomes

The collection of outcomes is a list. Sometimes a race condition causes to instances
of the same outcome to get added to the list shown in the report. By replacing this
with a set and propery equals/hashCode, duplicates are prevented from appearing
in the report.

I added test cases to prove that that POJO is properly managed inside a Set and also
to show that duplicates don't appear in the final report.