public void SystemsExecutesIFixedUpdateSystemSystem()
        {
            var system = new FixedUpdateSystemSpy();

            _systems.Add(system);
            _systems.FixedUpdate();

            Assert.AreEqual(1, system.DidExecute);
        }
        public void ExecutesFixedUpdateSystemSpy()
        {
            var system = new FixedUpdateSystemSpy();

            Assert.AreEqual(0, system.DidExecute);

            system.FixedUpdate();

            Assert.AreEqual(1, system.DidExecute);
        }