Add flag testmode_supported to sales channels (#1455)

* Add testmode-support-flag to SalesChannels

* Make saleschannels/testmode-warnings even more dangerous!

* Add warning for payment-methods that do support testmode but are being used in a non-testmode order caused by a saleschannel in a testmode-shop.

* Remove redundant testmode_supported-flag for WebshopSalesChannel

* Raise error on API when sales_channel does not support testmode

* Tests

* Fix style issue after merge

src/pretix/api/serializers/order.py

@@ -675,6 +675,20 @@ class OrderCreateSerializer(I18nAwareModelSerializer):
             raise ValidationError(errs)
         return data
 
+    def validate_testmode(self, testmode):
+        if 'sales_channel' in self.initial_data:
+            try:
+                sales_channel = get_all_sales_channels()[self.initial_data['sales_channel']]
+
+                if testmode and not sales_channel.testmode_supported:
+                    raise ValidationError('This sales channel does not provide support for testmode.')
+            except KeyError:
+                # We do not need to raise a ValidationError here, since there is another check to validate the
+                # sales_channel
+                pass
+
+        return testmode
+
     def create(self, validated_data):
         fees_data = validated_data.pop('fees') if 'fees' in validated_data else []
         positions_data = validated_data.pop('positions') if 'positions' in validated_data else []