Manually assign `notification_email` in the User factory when stubbed

Because we assign this value in the model via a callback conditionally on email_changed?, this never gets set when using build_stubbed, resulting in a "can't be blank" validation error on this field.

In this case, we can just assign it manually to the same value as email, which is generated via a sequence.

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13356#note_37088331