CI/TST: Call join on server process test (#45628)

Author: mroeschke

pandas/tests/io/test_user_agent.py

@@ -218,6 +218,7 @@ def responder(request):
     )
     server_process.start()
     yield port
+    server_process.join(10)
     server_process.terminate()
     kill_time = 5
     wait_time = 0