test: update source mock to use mockgen

What does this MR do?

Closes #277 (closed)

TODO

• I added the Changelog (e.g. Changelog: feature) trailer to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines