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
Here's an example of where you are doing an assert on the `name`, which is great, but the name will be the same for any task created from a Mock, so it would be a bit more accurate if the Mock created a task with a unique name (which I mentioned above in the code for the Mock).