Performancefilter - improve sorting

Ordering of Pairs without history should remain identical, so pairs with
positive performance move to the front, and negative pairs move to the back.

closes #4893

tests/conftest_trades_usdt.py

@@ -342,7 +342,8 @@ def mock_trade_usdt_7(fee):
         stake_amount=20.0,
         amount=2.0,
         amount_requested=2.0,
-        open_date=datetime.now(tz=timezone.utc) - timedelta(minutes=5),
+        open_date=datetime.now(tz=timezone.utc) - timedelta(minutes=20),
+        close_date=datetime.now(tz=timezone.utc) - timedelta(minutes=5),
         fee_open=fee.return_value,
         fee_close=fee.return_value,
         is_open=False,