Make tests work from anywhere

Right now the KataGo-custom tests only work when running from the root of the repo (see https://github.com/HumanCompatibleAI/KataGo-custom/blob/stable/.circleci/config.yml) It would be nice if we could get it to work when you run from anywhere, though this might be tricky given we compile the tests into an executable. Worth investigating more at least.