text: Title: Support for Binance Futures Testnet
  Body: I'm trying to modify the source to add support for Binance Futures Testnet.  
  
  I have made the following changes in `cryptofeed/exchanges/binance_futures.py`:
  - Line 49: changed `self.ws_endpoint` from `'wss://fstream.binance.com'` to `'wss://stream.binancefuture.com'`.
  - Line 50: changed `self.rest_endpoint` from `'https://fapi.binance.com/fapi/v1'` to `'https://testnet.binancefuture.com/fapi/v1'`.
  
  But none of the pubic feeds (_e.g._ trades) work.
  
  Can anyone help?