ModelStateDictionaryExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web.Http;

public static class ModelStateDictionaryExtensions
{
    public static IEnumerable<string> ToErrors(this ModelStateDictionary dict)
    {
        return dict.OrderBy(kvp => kvp.Key)
                   .SelectMany(kvp => kvp.Value.Errors.Select(e => Tuple.Create(kvp.Key, e.ErrorMessage)))
                   .Select(tuple => String.Format("{0} = {1}", tuple.Item1, tuple.Item2));
    }
}

SampleTest.cs

[Fact]
public void SampleTest()
{
    var modelStateDictionary = SomethingUnderTest();
    
    Assert.Collection(modelStateDictionary.ToErrors(),
        error => Assert.Equal("key1 = The first value for key1"),
        error => Assert.Equal("key1 = The second value for key1"),
        error => Assert.Equal("key2 = The first value for key2")
    );
}