line: fee tests ergänzt

line.py

@@ -283,7 +283,10 @@ class Line(SecondUomMixin, metaclass=PoolMeta):
             ).select(
                 tab_line.id,
                 Sum(Coalesce(
-                    tab_inout_fee.credit - tab_inout_fee.debit,
+                    # out-booking, positive amount = fee positive
+                    tab_inout_fee.debit - tab_inout_fee.credit,
+                    # a category-out on splitbooking as counterpart of
+                    # transfer = fee is positive
                     tab_mv_spline_fee.amount,
                     Case(
                         (tab_line.bookingtype == 'spin', tab_spline_fee.amount),