Cartesian Product based testing

[adam-rocska]

At 21Gram for our TypeScript stuff we have a utility function that takes a recursive/nested set of test values and provides a callback that runs your test code based on the cartesian product of the inputs. It's pretty dope.

It'd be nice to have something like it in Swift too.

[adam-rocska]

Gotta keep an eye on this lib: https://github.com/swiftlang/swift-testing
Either I might contribute there, or have a general purpose cartesian product producer here... dunno, postponed for now until anyone needs it.