MonoBehaviourUnitTests.cs

public class MainMenuMonoBehaviourUnitTests
{
    [UnityTest]
    public IEnumerator MainMenu_CanStartGame()
    {
        var called = false;
        var sut = new GameObject().AddComponent<MainMenu>();
        sut.Interactions = new SpyMainMenuInteractions {OnStartGame = () => called = true};
        yield return new WaitForEndOfFrame();

        sut.StartGame();

        Assert.IsTrue(called);
    }
}