test: add ft_additional_exchange_init to simplify test setups

Author: xmatthias

freqtrade/exchange/exchange.py

@@ -300,7 +300,7 @@ def __init__(
 
         if self.trading_mode != TradingMode.SPOT and load_leverage_tiers:
             self.fill_leverage_tiers()
-        self.additional_exchange_init()
+        self.ft_additional_exchange_init()
 
     def __del__(self):
         """
@@ -455,6 +455,12 @@ def precision_mode_price(self) -> int:
         """
         return self._api.precisionMode
 
+    def ft_additional_exchange_init(self) -> None:
+        """
+        Wrapper around additional_exchange_init to simplify testing
+        """
+        self.additional_exchange_init()
+
     def additional_exchange_init(self) -> None:
         """
         Additional exchange initialization logic.