Fix `test_settlement.py` tests
Fix `test_settlement.py` tests

This fixes #390
with the exception of `test_start_end_attack` -- which should be fixed once we
have the new state machine task model.

tomashaber/raiden,hackaugusto/raiden,tomashaber/raiden,tomashaber/raiden,charles-cooper/raiden,tomashaber/raiden,hackaugusto/raiden,charles-cooper/raiden,tomashaber/raiden