When you generate tests for an interface, an new class is generated implementing the interface. While I do not use this feature myself, I did notice that this class only implements the methods described by the interface you are testing, NOT any methods described by the interfaces that the tested interface extends. The result is that uncompilable code is generated.
While a simple alt-enter -> 'implement all' resolves the issue, it is still an issue.
It is a minor issue in a barely ever used feature with a very simple work-around, so I gave it priory 4.

I did not test what happens with nested abstract classes, so it may be worth to also check those while fixing this bug (if it is worth fixing).