Were Bun's tests generated by an LLM? If they were, were they read by a human afterwards to be validated?
Neither of these can be replaced by an automated test suite of any kind, and all of these are examples of good engineering practices that guarantee software quality.
Additionally, even if you don't (need to) adhere to the best engineering practices and instead rely solely on an automated test suite, the tests in this suite must be validated - read and understood - by a human in order to guarantee that they nail down the correct requirements.