A good design is about having stark clarity as what the entity under test is and having a clear set of test cases for each one these entities.
In the context of agile, there are FOUR kinds of entities we can consider for testing as below.
E1 – A user story
E2 – A Collection of user stories
E3 – A usage flow that constitutes a set of user stories
E4 – Set of user stories across releases/sprints/epics – bigger flow
Note that the test cases get closer to an end-user scenario usage as we move from E1-E4.