Bug 20990: (QA follow-up) make outstanding_credits return the account lines only

Bug 20990: (QA follow-up) make outstanding_credits 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_credits return the account
lines, code that used the $total value, will just use

    $lines->total_outstanding;

Tests are adjusted accordingly.

To test:
- Run:
  $ kshell
 k$ prove t/db_dependent/Koha/Account.t
=> SUCCESS: Test pass.

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