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
The new method you have added above (`is_completed_button_visible`) suggests that this method is no longer valid, as all it's doing is checking the visibility of the element at `*self._get_started_button_locator`, but it looks like that button could have a label of `Completed` as opposed to `Get Started`, so this needs to be addressed.