I think you generally need to either assume or establish that your testing environment tends to be sound, just like you need to assume or establish that your programming language/compiler/computing environment tends to be sound. Your application code and test code itself probably shouldn't be compensating for or attempting to avoid known or unknown flaws in your testing environment.
How do you establish that the testing environment is sound besides having defensive tests? It doesn't necessarily have to be the case that all your tests are like that.
