So you just copy and paste the subsets of the Backend methods and their signature into separate interfaces that only have the methods needed for the user function and its test? That doesn't sounds very good.

I'd rather have one global "backend" interface and have mocks use that. If the type system gives me issues with unimplemented methods, than that's the problem with the type system itself -- allowing for partial interfaces (through keywords or something) should be a first-class feature.