Add helpers for mocking the API in tests

[carlobeltrame]

Maybe similar to https://github.com/ctimmerm/axios-mock-adapter
Maybe draw inspiration from https://github.com/ecamp/ecamp3/blob/1a1e33e8f89fdb0cb2241f15d7c6818812c82a19/frontend/src/components/form/api/__tests__/ApiMock.js