I tried this and found its working. I tried to pick a test story before implementation. Off course this should be discussed with team and Product Owner. If stories are big and can be divided into test and implementation story, then we try to do that and I pick the test story one sprint before the actual implementation. As per one of my friend, we can termed this test story as analysis story. But I would like to term it as test story.
The benefits I saw:
1. After I finished with my test design (Specification by example), I did a review with my development team, and this helped my team to really consider and think all possible constrains and helped them to understand the requirement better. Everybody brought different question which needs business clarification.
Now imagine, this could have been a disaster if it comes up during implementation.
2. when we asked clarification from business, (we also had test scenario review with business) they also had to given a rethink about their requirement. Because of the test scenarios, business is able to see the impact of the requirement. Business had to redefine their requirement which is now more polished.
3. Finally in the sprint I was able to help my development team and business with my test scenarios, able to bring business and development team in the same direction.
4. Sprint after when the actual implementation happened, it was smooth and we had quality output at the end. Without any trouble team developed the functionality, we had a good test results.