test(background): fix locust quitting handler signature so job accounting prints

The events.quitting listener took _environment as a required positional
arg, but locust passes environment as a keyword, so the handler raised
TypeError at shutdown and the submitted/completed summary never printed
(locust exited 2). Accept the event kwargs via **_kwargs.

src/backend/tests/locust/v2_background_locustfile.py

@@ -148,7 +148,7 @@ class BackgroundWorkflowUser(FastHttpUser):
 
 
 @events.quitting.add_listener
-def _print_counts(_environment, **_kwargs) -> None:
+def _print_counts(**_kwargs) -> None:
     with _lock:
         snap = dict(COUNTS)
     total = snap["submitted"] or 1