Results 2 issues of Johan

In order to facilitate unit tests on business logic, we should provide a mockFirebaseSource. We can inspire on mockDOMSource: https://github.com/cyclejs/dom/blob/master/src/mock-dom-source.js

Seems like update showed version doesn't update the version comment ![image](https://github.com/notum-cz/strapi-plugin-content-versioning/assets/540172/11a0f772-b633-4405-98a9-3dcdd04915cc) The only way to create or modify a version comment is by creating a new version. Seems like this...