New Post: Testing an "InEnumValidator" custom validator

I created a custom validator, which tests that an enum is valid, that means it is within the enum's range of valid values:
  [DebuggerStepThrough]
  public class IsInEnumValidator<T> : PropertyValidator {
    public IsInEnumValidator() : base("Property {PropertyName} it not a valid enum value.") { }
    protected override bool IsValid(PropertyValidatorContext context) {
      if (!typeof(T).IsEnum) return false;
      return Enum.IsDefined(typeof(T), context.PropertyValue);
    }
  }

And an extension method for chaining validators:
    [DebuggerStepThrough]
    public static IRuleBuilderOptions<T, TProperty> IsInEnum<T, TProperty>(this IRuleBuilder<T, TProperty> ruleBuilder) {
      return ruleBuilder.SetValidator(new IsInEnumValidator<TProperty>());
    }

To use it:
RuleFor(x => x.Day).IsInEnum<DayOfWeek>();

Now I need to test the validator- not test my data with the validator, but to test the validator itself. The library has quite a bit of stuff going on, so I can't figure out how to do it. Can someone give me a pointer? I use NUnit.