If your code and tests are so opaque that the reviewer can't understand them without physically running and modifying the code, then as a reviewer you should as

If your code and tests are so opaque that the reviewer can't understand them without physically running and modifying the code, then as a reviewer you should ask the author to make it clearer. Because otherwise that's how you get write-only code. Things like code style and documentation can make a big difference, and review time is when you can help enforce them and build team culture and knowledge.