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
I'm concerned that this duplicates a lot of code from `base/login()` [1], and it also has an issue that we discussed awhile back when writing the `login` method which is that this code expects that the next page will be the `TaskDetailsPage` but that will not necessarily always be true. There may be other places where a user is asked to log in and is then redirected to a different page.

I would suggest trying to find a way to reuse the `login` method from `Base` which could address both of these issues.

[1] https://github.com/mozilla/oneanddone-tests/blob/master/pages/base.py#L26