code: def llm(self, output: Optional[str] = None):
      return FakeLLM(output=output)
  
imports: import json
  import os
  import time
  from typing import Optional
  from unittest.mock import MagicMock, Mock, patch
  import pandas
  import pytest
  from pandasai.helpers.query_exec_tracker import QueryExecTracker
  from pandasai.llm.fake import FakeLLM
  from pandasai.smart_dataframe import SmartDataframe
  from unittest import TestCase
  from datetime import datetime, timedelta
label_desc: Use image node_id 1 for calling the TestQueryExecTracker obj's underlying member method code with example usage: obj.llm(output) and returns: FakeLLM