test: drain proxy timeout before shutdown

scripts/verifyProxy.py

@@ -233,6 +233,11 @@ def main() -> None:
                 ) from error
             verify_keep_alive(edge_port)
             verify_timeout(edge_port)
+            # The upstream /slow handler intentionally runs for two seconds.
+            # Nginx has already returned 504, but it may keep the upstream
+            # socket around while native drains that request. Let it finish
+            # before checking a clean listener shutdown.
+            time.sleep(2.2)
             print("PASS Nginx TLS proxy, Keep-Alive and upstream timeout", flush=True)
 
             assert backend.stdin is not None