public void WhenContainsNonRomanNumeralChar_ThenThrowException()
 {
     Assert.Throws <ArgumentException>(() => RomanNumeralConverter.ToInt32("MPI"));
 }
            public void WhenValid_ThenReturnInt(string numeral, int expected)
            {
                var result = RomanNumeralConverter.ToInt32(numeral);

                Assert.That(result, Is.EqualTo(expected));
            }
 public void WhenIsNullOrEmpty_ThenThrowException(string numberal)
 {
     Assert.Throws <ArgumentException>(() => RomanNumeralConverter.ToInt32(numberal));
 }