text: Title: [Feature]: Overall tests improvement and speedup
  Body: ### 🚀 The feature, motivation and pitch
  
  Currently, vLLM test suite are often very expensive and takes a long time to setup and run (approx 1h for the full suite).
  
  Majority of these times spent are at our frontend code.
  
  Given that API logics are relatively complex (OpenAI vs. API server), we can speed up by mocking and keep e2e to a minimum.
  
  ### Alternatives
  
  _No response_
  
  ### Additional context
  
  _No response_
  
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