Add clearMocks Jest config

**Why?**
This removes the need to add `jest.clearAllMocks` manually.
Without this, it's possible for tests to have false positives
because the `mock.calls` is never reset.

**References:**
- https://jestjs.io/docs/en/configuration#clearmocks-boolean
- https://gitlab.com/gitlab-org/gitlab/merge_requests/21860