The scrum guide says: "A new Sprint starts immediately after the conclusion of the previous Sprint."

you don't want your unit test to really make changes so it cannot exercise the real persistence layer.

A unit test is a test that isolates and verifies the functionality of one unit.

The sprint is the heartbeat of scrum, during the sprint the increment is created. The increment must be usable and all the work that's part of the increment should adhere to the definition of done.

Detailed Explanation: Scrum promotes self-management by empowering the team to decide what work they take on, removing hierarchical titles to foster equality and collaboration, and having the Scrum Master shield the team from external disruptions so they can focus on their goals.