¿Cómo puedo probar un método que usa un objeto de sesión dentro de su cuerpo?

Digamos que tengo la siguiente acción:

[HttpPost]
public JsonResult GetSearchResultGrid(JqGridParams gridParams, Guid campaignId, string queryItemsString)
{
    var queryItems = new JavaScriptSerializer().Deserialize<IList<FilledQueryItem>>(queryItemsString);
    IPageData pageData = gridParams.ToPageData();
    var extraFieldLinker = SessionHandler.CurrentExtraFieldsLinker;
    var searchParams = new SearchParamsModel(extraFieldLinker, queryItems);
    IList<CustomerSearchResultRow> searchResults = null;
    searchResults = _customerService.SearchCustomersByUrlAndCampaign(campaignId,
        searchParams.SearchString,
        searchParams.AddressFilterPredicate,
        pageData);
    return GetGridData<CustomerSearchResultGridDefinition, CustomerSearchResultRow>(searchResults, pageData);
}

Hice las siguientes pruebas unitarias que fallaron hasta ahora debido a la sesión:

[Test]
public void CanGetSearchResultGrid()
{
    //Initialize
    var mockJqGridParams = new Mock<JqGridParams>();
    var mockPageData = new Mock<IPageData>();
    IPagedList<CustomerSearchResultRow> mockPagedResult = new PagedList<CustomerSearchResultRow>(mockPageData.Object);
    var guid= Guid.NewGuid();
    const string searchString =
        "[{\"Caption\":\"FirstName\",\"ConditionType\":\"contains\",\"Value\":\"d\",\"NextItem\":\"Last\"}]";
    Func<Address,bool> addressFilterPredicate = (x => true);

    //Setup
    mockJqGridParams.Setup(x => x.ToPageData()).Returns(mockPageData.Object);
    _customerService.Setup(x => x.SearchCustomersByUrlAndCampaign(guid, searchString, addressFilterPredicate, mockPageData.Object))
        .Returns(mockPagedResult);

    //Call
    var result = _homeController.GetSearchResultGrid(mockJqGridParams.Object, guid, searchString);

    mockJqGridParams.Verify(x => x.ToPageData(), Times.Once());
    _customerService.Verify(x => x.SearchCustomersByUrlAndCampaign(guid, searchString, addressFilterPredicate, mockPageData.Object)
        , Times.Once());

    //Verify
    Assert.That(result, Is.Not.Null);
    Assert.That(result, Is.TypeOf(typeof(JsonResult)));
}

Y el método del ayudante, por supuesto:

   public static ExtraFieldsLinker CurrentExtraFieldsLinker
    {
        get
        {
            object extraFieldLinker = GetSessionObject(EXTRA_FIELDS_LINKER);
            return extraFieldLinker as ExtraFieldsLinker;
        }
        set { SetSessionObject(EXTRA_FIELDS_LINKER, value); }
    }