[Enhancement] Use MongoDB Mock for Testing

Summary

At the moment, the testing code requires a live mongo database. This complicates automated testing and CI/CD quite a lot. Instead, we could switch to database mocks. See for example here:

• https://medium.com/@victor.neuret/mocking-the-official-mongo-golang-driver-5aad5b226a78
• https://liza.io/how-im-testing-mongodb-calls-in-go-for-now/

However, we probably can use testify/mock.

Why is the current feature not sufficient?

• Not really compatible with CI
• More work setting up local dev setup