The way to test an abstract class is to test one or more simple concrete examples of that class. For Game::Die you would test a few easy types of die: 1d6, 1d10, 2d20, etc. For Game::AI, you may have to make a tiny example game that uses all the features of an AI. It doesn't have to be a fun game, or a real game. Find a Tic Tac Toe game and implement that AI.

I don't know what the allure for XS would be in a minimax framework. It's so thin, and won't be the real work of any minimaxable game. Build a tree of foreseen board positions, using one AI per player. One method on AI can be coded to simply return a list of all legal possible moves, instead of rushing to commit the actual board to a single move. Then for any given board position, you have to rank the "value" of that position or the move that achieved it. The only real "work" involved would be to weed the tree of disastrous courses of action before you spent a ton of time asking the AI to think farther along those lines.

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