If a class is only used internally by another class, don’t test it directly. Test it through the public class it serves. If HideMe is private to CallMe and you refactor their internal API, HideMe’s tests break without adding information that CallMe’s tests don’t already provide. Those tests have zero marginal value and non-zero maintenance cost.
Aggressively push logic into private classes, limit the public surface that needs direct testing, and verify through the public contract. This yields high accuracy with reasonable completeness and a smaller, more maintainable suite.