public void ParseIntReturnsInt()
        {
            sut = new Calculator();

            Assert.That(sut.GetValidIntFromString("5"), Is.EqualTo(5));
        }
        public void ParseIntReturnsZeroWhenNonDigit()
        {
            sut = new Calculator();

            Assert.That(sut.GetValidIntFromString("a"), Is.EqualTo(0));
        }