Due to the similarities between React and Preact (including the testing libraries), you should be able to comfortably use any React based examples, docs, answers on stack overflow etc.
Take note of the differences between React and Preact.
If you're still hungry for more at this point than checkout:
- The dom-testing-library:
- The react-testing-library:
- This YouTube video by LevelUpTuts called What is React Testing Library?
- Extending Jest with custom matchers to test the state of the DOM.