public void Should_ReturnNotValidRomanNumeralError_When_InputHasCharactersOtherThanValidRomanNumerals(string romanNumeral)
        {
            const string errorMessage = "Input is invalid - one or more input characters are not a valid Roman numeral";

            var result = _validator.Validate(romanNumeral);

            result.Messages.Should().Contain(errorMessage);
        }