SitecoreFieldCollectionMock.cs

public FieldCollection CreateFieldCollection()
{
    var db = Substitute.For<Database>();
    var item = Substitute.For<Item>(ID.NewID, ItemData.Empty, db);
    var fieldCollection = Substitute.For<FieldCollection>(item);
    return fieldCollection;
}

public FieldCollection CreateFieldCollectionByDatabase(Database db)
{
    var item = Substitute.For<Item>(ID.NewID, ItemData.Empty, db);
    var fieldCollection = Substitute.For<FieldCollection>(item);
    return fieldCollection;
}

public FieldCollection CreateFieldCollectionByItem(Item item)
{
    var fieldCollection = Substitute.For<FieldCollection>(item);
    return fieldCollection;
}