How to test transformations without real data?

Imagine an ordinary situation: you are creating and debugging your dbt transformations in a developer environment using a real DB schema. You want to create tests, but you have no real data because it is all on the production environment, and it is sensitive. What do people usually do in this situation?

You could manually create seeds to stand in for the sources, or use a tool like this to help do that: dbt - Package hub (