chore: rename testlab sample fn

Author: Salakar

samples/basic_test_lab/functions/main.py

@@ -7,7 +7,7 @@
 
 
 @on_test_matrix_completed()
-def handle_test_matrix_completed(
+def testmatrixcompleted(
         event: CloudEvent[TestMatrixCompletedData]) -> None:
     print(f"Test Matrix ID: {event.data.test_matrix_id}")
     print(f"Test Matrix State: {event.data.state}")