Minor fix to an assertion message (#34861)

Author: cecille

src/python_testing/matter_testing_support.py

@@ -330,7 +330,7 @@ def wait_for_report(self):
             logging.info(
                 f"[AttributeChangeCallback] Got attribute subscription report. Attribute {path.AttributeType}. Updated value: {attribute_value}. SubscriptionId: {transaction.subscriptionId}")
         except KeyError:
-            asserts.fail("[AttributeChangeCallback] Attribute {expected_attribute} not found in returned report")
+            asserts.fail(f"[AttributeChangeCallback] Attribute {self._expected_attribute} not found in returned report")
 
 
 def await_sequence_of_reports(report_queue: queue.Queue, endpoint_id: int, attribute: TypedAttributePath, sequence: list[Any], timeout_sec: float):