Findall is just a cartesian product...something easily available in pretty much any language out there. Not that I think Prolog is a bad fit for test data generation, just that this example is entirely too simple to show its power.

Where Prolog becomes really powerful for this use case is modeling the underlying business rules and selectively creating test cases that acknowledge those rules. A tiny bit of familiarity with Prolog goes a long way here. I've modeled about 95% of the underlying logic of 50k SLOC Java business rule systems using less than a hundred lines of Prolog. And the ability to infer both forward and backwards is extremely powerful (ex. if you have created F(X,Y) = Z, then you implicitly have a rule for determining X given Y and Z).