My experience is in medical equipment -- not quite the mess that medical devices are, thankfully. The general idea is a combination of redundancy, clean-room methods, and people whose domain expertise is in testing and edge case analysis.
From a personal perspective, I find that trying to design tests as the developer of the code to be tested is fundamentally difficult -- I have embedded assumptions around the functionality of the code that make it cognitively difficult to fully envision complex failure scenarios. Some of the best test producers I've worked with have had no development background at all.