I was reviewing some coordinate conversion code one time and a developer’s method of creating unit tests was to run their code, generate a result, then make that output the correct result for the test. Except the result was wrong based on other utilities doing the same conversion, and also failed edge cases.

For the inexperienced dev audience: generally speaking that is a valid way to generate regression tests and document novel edge case behavior.

In your case of course any existing utilities behavior/specs should be tested first, regression tests just on top, if they are not already covered.