Enhance and fix tests for default discount modules
Flectra provides following default discount modules:
Current tests do only test one way, the outgoing way (outgoing invoice, sale order).
Steps to reproduce
As it are tests, no way to reproduce manually.
What is the current bug behavior?
Test are only check outgoing discount calculation.
What is the expected correct behavior?
Correct discount calculation for refunds.
Relevant logs and/or screenshots
In this MR's calculation in code is partially or fully fixed
Tests in above module must be fixed to test all, in and out discount calculation.