Previous tests did several risk operations on tests, like performing an unbound search
on payment orders, what can lead to errors if there are some data on DB (or for example
we add in the future some payment orders in demo.
This commit also split the tests for inbound payment in a base inheritable class that
can be reused in other dependant modules, and the test of this module.
Finally, we use SavepointCase for improving performance in tests.
context of payment orders, she should ocus on the amount that
is due to be paid. In this method we are forcing to display both
the amount due in company and in the invoice currency.
We then hide the fields debit and credit, because they add no value.
