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.