Unit Test a submodule static function in iOS? (Swift 4 - Xcode)

I have 4 submodules in my iOS project (Project A).

In one submodule (X Submodule) I have a class with a static function that get as input an Object defined in another submodule (Y submodule) and return a String.

How and where do I write a unit test for this static function logic? Do I write the logic in one of the App projects (A, B, C iOS Projects) that re-use those submodules (X, Y submodules)?

If you have any suggestion it will be highly appreaciated!