在我的(Swift)单元测试中尝试将获取的NSManagedObject向下转换为其正确的子类StoryPhotoTrack时，出现运行时错误:Couldnotcastvalueoftype'StoryPhotoTrack_StoryPhotoTrack_'(0x7fbda4734490)到“StoryPhotoTrack”(0x11edd1b08)。这是我失败/崩溃的测试:functestFetchingStoryWithCastingInvolved(){letstory:StoryPhotoTrack?=storyFetcher.fetchAnyStoryPhotoTrack()