Faster tests by not creating covers unless mandatory

api/funkwhale_api/federation/factories.py

api/funkwhale_api/music/factories.py

api/tests/audio/test_serializers.py

api/tests/audio/test_spa_views.py

api/tests/audio/test_views.py

api/tests/common/test_models.py

api/tests/common/test_tasks.py

api/tests/common/test_utils.py

api/tests/favorites/test_favorites.py

api/tests/federation/test_routes.py

api/tests/federation/test_serializers.py

api/tests/manage/test_serializers.py

api/tests/music/test_mutations.py

api/tests/music/test_serializers.py

api/tests/music/test_spa_views.py

api/tests/music/test_tasks.py

api/tests/music/test_views.py

api/tests/playlists/test_serializers.py

api/tests/playlists/test_views.py

api/tests/radios/test_api.py