SitecoreFieldMock.cs

public Field CreateField()
{
    var db = Substitute.For<Database>();
    var item = Substitute.For<Item>(ID.NewID, ItemData.Empty, db);
    var field = Substitute.For<Field>(ID.NewID, item);
    return field;
}

public Field CreateFieldByDatabase(Database db)
{
    var item = Substitute.For<Item>(ID.NewID, ItemData.Empty, db);
    var field = Substitute.For<Field>(ID.NewID, item);
    return field;
}

public Field CreateFieldByItem(Item item)
{
    var field = Substitute.For<Field>(ID.NewID, item);
    return field;
}