Put @keras_serializable only on layers it works on

And only run the test on TF*MainLayer classes so marked.

tests/test_modeling_tf_common.py

@@ -103,11 +103,9 @@ class TFModelTesterMixin:
             if module_member_name.endswith("MainLayer")
             for module_member in (getattr(module, module_member_name),)
             if isinstance(module_member, type) and tf.keras.layers.Layer in module_member.__bases__
+            and getattr(module_member, '_keras_serializable', False)
         )
         for main_layer_class in tf_main_layer_classes:
-            if main_layer_class.__name__ == "TFT5MainLayer":
-                # Not really a “main layer” as in the other models, as this one doesn't receive the test inputs directly
-                continue
             main_layer = main_layer_class(config)
             symbolic_inputs = {
                 name: tf.keras.Input(tensor.shape[1:], dtype=tensor.dtype) for name, tensor in inputs_dict.items()