code: def GET_confirmed_balance_insight( self, path_info, address ):
          
          if not BLOCKSTACK_TEST:
              return self._send_headers(status_code=404, content_type='text/plain')
  
          if not check_address(address):
              return self._reply_json({'error': 'Invalid address'}, status_code=400)
  
          res = self._get_balance(address, 1)
          if 'error' in res:
              return self._reply_json(res, status_code=502)
  
          return self._reply_json(res['balance'])
docs: Works only in test mode!
          Handle GET /insight-api/addr/:address/balance