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
Although it's nice to have coverage for the `LoginRequiredPage`, this test in particular does not need to exercise that, so you could simplify it by just having the user log in at the beginning of the test and not worry about the `LoginRequiredPage` at all. This would also eliminate the need to address my comment at https://github.com/mozilla/oneanddone-tests/pull/16/files#discussion_r15885659.