Bug 1038618 - [testing] Test that one time tasks are marked completed on...

The test works fine on merging https://github.com/mozilla/oneanddone/pull/163
It might be worth doing something similar to what we do in `MockUser` where we incorporate a timestamp into some of the attributes, which will allow us to uniquely identify the task during a test, both to differentiate between tasks if we create more than one, and to assert that we are interacting with the task with which we expect to interact.