If you don't trust the person to test their code, what really can you trust them with? If you can't trust them to do that, do you honestly think that you can trust them to do any post-code review follow ups?
Sure, one can say that just because they tested it, does not mean that they tested it well. But that leads to a different question--do you trust them to test their code well?
If you have concerns about how well it was tested, think up all the failure paths you can think of, and all the corner cases and keep an eye out for those in the code review, or during your pre-code review inspection (make notes). If you have concerns about the cases you have thought up, ask about them. If they can not be answered to satisfaction at the session, then the developer must follow up on them; make sure to be copied on the results of the investigation.