[C#]privateメソッドの引数のenumをリフレクションでテストに使う

Class1.cs

using System;

namespace sample
{
    public class Class1
    {
        private string GetValue(MyEnum em)
        {
            if (Enum.IsDefined(typeof(MyEnum), em))
            {
                return ((MyEnum)em).ToString();
            }
            return string.Empty;
        }
    }

    enum MyEnum
    {
        first,
        second,
        third,
    }
}

UnitTest1.cs

using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using System.Reflection;

namespace sample.Test
{
    [TestClass]
    public class UnitTest1
    {
        [TestMethod]
        public void TestMethod1()
        {
            var a = Assembly.Load("sample");
            var type = a.GetType("sample.MyEnum");
            //var underlyingType = Enum.GetUnderlyingType(type);
            var enumValues = Enum.GetValues(type);

            var value = enumValues.GetValue(0);
            //var ret = Convert.ChangeType(value, underlyingType);

            var target = new Class1();
            var po = new PrivateObject(target);

            var arg = Convert.ChangeType(value, type);
            var args = new object[] { arg };

            var expected = "first";
            var actual = po.Invoke("GetValue", args);

            Assert.AreEqual(expected, actual);
        }
    }
}