Fix mock_tools import

Author: bcherry

tests/test_agent.py

@@ -1,6 +1,5 @@
 import pytest
-from livekit.agents import AgentSession, llm
-from livekit.agents.voice.run_result import mock_tools
+from livekit.agents import AgentSession, llm, mock_tools
 from livekit.plugins import openai
 
 from agent import Assistant