Bug 20946: (QA follow-up) make outstanding_debits return the account lines only

Bug 20946: (QA follow-up) make outstanding_debits return the account lines only

This patch was discussed with Jonathan on a QA conversation. It is
better to keep this simpler and more reusable. And is the right approach
in this case.

This patch makes Koha::Account::outstanding_debits return the account
lines, and a method is added to Koha::Account::Lines so the outstanding
amount is calculated on the resultset. This is done the dame way it was
done before, and the tests got adjusted.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>