Bug 21969: Regression tests

Bug 21969: Regression tests

This patch introduces regression tests for Koha::Account::outstanding_*
methods so they don't pick wrong lines when amountoutstanding matches
what we are looking for (i.e. negative for credits and positive for
debits).

To test:
- Apply this patch
- Run:
  $ kshell
 k$ prove t/db_dependent/Koha/Account.t
=> FAIL: Tests fail because pathological account lines are wrongly
picked.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>