However, couldn't you just generate the missing methods at compile time (assuming there is some kind of introspection on your classes interfaces)?

In theory, yes. But that's still going to be adding more complexity to your testing code that isn't directly related to the job of making sure the stuff under test behaves as expected. Sure you can hide it away in a superclass, but it will slow you down. And getting it right is likely to be complicated enough that you're going to have to write tests for your tests to make sure that interfaces are getting stubbed out with sensible methods.