Unit testing methods using AddOrUpdate with Entity

I've managed to Mock an Entity Framework dbcontext and dbset to allow for unit testing querying functions against a repository component.

I've been unable to perform a successful test against an a method using Entity Frameworks' AddOrUpdate() method. The error received is:

"Unable to call public, instance method AddOrUpdate on derived IDbSet type 'Castle.Proxies.DbSet`1Proxy'. Method not found."

Is it at all possible to test this?

    private IRepository _Sut;
    private Mock<DbSet<JobListing>> _DbSet;
    private Mock<RecruitmentDb> _DbContext;

    [SetUp]
    public void Setup()
    {
        _DbContext = new Mock<RecruitmentDb>();

        var JobsData = GenerateJobs().AsQueryable();

        _DbSet = new Mock<DbSet<JobListing>>();
        _DbSet.As<IQueryable<JobListing>>().Setup(x => x.Provider).Returns(JobsData.Provider);
        _DbSet.As<IQueryable<JobListing>>().Setup(x => x.Expression).Returns(JobsData.Expression);
        _DbSet.As<IQueryable<JobListing>>().Setup(x => x.ElementType).Returns(JobsData.ElementType);
        _DbSet.As<IQueryable<JobListing>>().Setup(x => x.GetEnumerator()).Returns(JobsData.GetEnumerator());

        _DbContext.Setup(x => x.JobListings).Returns(_DbSet.Object);
        _Sut = new JobListingRepository(_DbContext.Object);
    }

    [Test]
    public void Update_ChangedTitleProperty_UpdatedDetails()
    {
        var Actual = GenerateJobs().First();
        var OriginalJob = Actual;
        Actual.Title = "Newly Changed Title";
        _Sut.Update(Actual);

        Actual.Title.Should().NotBe(OriginalJob.Title);
        Actual.Id.Should().Be(OriginalJob.Id);
    }

    private List<JobListing> GenerateJobs()
    {
        return new List<JobListing>
        {
            new JobListing{ Id = 1,
            Title = "Software Developer",
            ShortDescription = "This is the short description",
            FullDescription = "This is the long description",
            Applicants = new List<Applicant>(),
            ClosingDate = DateTime.Now.AddMonths(5).Date},

            new JobListing{
            Id = 2,
            Title = "Head Chef",
            ShortDescription = "This is the short description",
            FullDescription = "This is the long description",
            Applicants = new List<Applicant>(),
            ClosingDate = DateTime.Now.AddMonths(2).Date
            },

            new JobListing
        {
            Id = 3,
            Title = "Chief Minister",
            ShortDescription = "This is the short description",
            FullDescription = "This is the long description",
            Applicants = new List<Applicant>(),
            ClosingDate = DateTime.Now.AddMonths(2).Date
        }
        };
    }

The issue is because AddOrUpdate is an extension method. I overcame this issue by designing a wrapper. You can mock/stub the IAddOrUpdateHelper interface instead.

    public class AddOrUpdateHelper : IAddOrUpdateHelper
    {

        public void AddOrUpdateEntity<TEntity>(DataContext db, params TEntity[] entities) where TEntity : class
        {
            db.Set<TEntity>().AddOrUpdate(entities);
        }
    }

    public interface IAddOrUpdateHelper
    {
        void AddOrUpdateEntity<TEntity>(DataContext db, params TEntity[] entities) where TEntity : class;
    }