Test removing a non-existent artist (TestRemoveArtist.test_remove_nonexistent_artist)
Test adding a concert without specifying an artist (TestAddConcert.test_add_concert_without_artist)
Test removing a non-existent concert (TestRemoveConcert.test_remove_nonexistent_concert)
Test removing a non-existent artist (TestRemoveArtist.test_remove_nonexistent_artist) Test adding a concert without specifying an artist (TestAddConcert.test_add_concert_without_artist) Test removing a non-existent concert (TestRemoveConcert.test_remove_nonexistent_concert)