Fixed a bug in event_based/core_test.py due to the version of mock used

Sorry, but it turns out I was using mock 1.0, whereas the Ubuntu version of mock is 0.7.2, so I did not see this problem. I also think Luigi is using a newer mock, so he did not see the problem either. I have changed my system to use the Ubuntu official version.

The problem was introduced when fixing https://bugs.launchpad.net/oq-engine/+bug/1167310