Fix weird broken test that never ran (#827)

Just stumbled across this, we had a test that didn't have test in the
name, was never run, and was very broken if you tried to run it.

keras_core/saving/serialization_lib_test.py

@@ -49,18 +49,10 @@ def get_config(self):
         }
 
 
-class WrapperLayer(keras_core.layers.Layer):
-    def __init__(self, layer, **kwargs):
-        super().__init__(**kwargs)
-        self.layer = layer
-
+class WrapperLayer(keras_core.layers.Wrapper):
     def call(self, x):
         return self.layer(x)
 
-    def get_config(self):
-        config = super().get_config()
-        return {"layer": self.layer, **config}
-
 
 class SerializationLibTest(testing.TestCase):
     def roundtrip(self, obj, custom_objects=None, safe_mode=True):
@@ -210,20 +202,22 @@ def test_dict_inputs_outputs(self):
         self.assertAllClose(original_output["foo"], restored_output["foo"])
         self.assertAllClose(original_output["bar"], restored_output["bar"])
 
-    def shared_inner_layer(self):
-        input_1 = keras_core.Input((2,))
-        input_2 = keras_core.Input((2,))
-        shared_layer = keras_core.layers.Dense(1)
-        output_1 = shared_layer(input_1)
-        wrapper_layer = WrapperLayer(shared_layer)
-        output_2 = wrapper_layer(input_2)
-        model = keras_core.Model([input_1, input_2], [output_1, output_2])
-        _, new_model, _ = self.roundtrip(
-            model, custom_objects={"WrapperLayer": WrapperLayer}
-        )
+    @pytest.mark.requires_trainable_backend
+    def test_shared_inner_layer(self):
+        with serialization_lib.ObjectSharingScope():
+            input_1 = keras_core.Input((2,))
+            input_2 = keras_core.Input((2,))
+            shared_layer = keras_core.layers.Dense(1)
+            output_1 = shared_layer(input_1)
+            wrapper_layer = WrapperLayer(shared_layer)
+            output_2 = wrapper_layer(input_2)
+            model = keras_core.Model([input_1, input_2], [output_1, output_2])
+            _, new_model, _ = self.roundtrip(
+                model, custom_objects={"WrapperLayer": WrapperLayer}
+            )
 
-        self.assertIs(model.layers[2], model.layers[3].layer)
-        self.assertIs(new_model.layers[2], new_model.layers[3].layer)
+            self.assertIs(model.layers[2], model.layers[3].layer)
+            self.assertIs(new_model.layers[2], new_model.layers[3].layer)
 
     @pytest.mark.requires_trainable_backend
     def test_functional_subclass(self):