Discounts: Fix edge case in computation (#4126)

* Add new test case for discounts:

Two discounts:
- "For every 1 item1, you get three item2 for 10 % off."
- "For every 1 item1, you get five item3 for 10 % off."
Cart: 2x item1, 2x item2, 6x item3
Expected result: 2x item1 full price, 2x item2 discounted, 5x item3 discounted, 1x item3 full price

* Fix discount calculation bug

* Update src/pretix/base/models/discount.py

Co-authored-by: Raphael Michel <michel@rami.io>

* Update src/pretix/base/models/discount.py

Co-authored-by: Richard Schreiber <schreiber@rami.io>

---------

Co-authored-by: Raphael Michel <michel@rami.io>
Co-authored-by: Richard Schreiber <schreiber@rami.io>

src/pretix/base/models/discount.py

@@ -23,6 +23,7 @@
 from collections import defaultdict
 from decimal import Decimal
 from itertools import groupby
+from math import ceil
 from typing import Dict, Optional, Tuple
 
 from django.core.exceptions import ValidationError
@@ -272,7 +273,7 @@ class Discount(LoggedModel):
 
             # Prevent over-consuming of items, i.e. if our discount is "buy 2, get 1 free", we only
             # want to match multiples of 3
-            n_groups = min(len(condition_idx_group) // self.condition_min_count, len(benefit_idx_group))
+            n_groups = min(len(condition_idx_group) // self.condition_min_count, ceil(len(benefit_idx_group) / self.benefit_only_apply_to_cheapest_n_matches))
             consume_idx = condition_idx_group[:n_groups * self.condition_min_count]
             benefit_idx = benefit_idx_group[:n_groups * self.benefit_only_apply_to_cheapest_n_matches]
         else: