Bug 21909: Unit tests

Bug 21909: Unit tests

This patch introduces regression tests for the behaviour to be
introduced. Both outstanding_credits and outstanding_debits methods
should return a Koha::Account::Lines object in scalar context, and a
list of Koha::Account::Line objects in list context.

To test:
- Apply this patch
- Run
  $ kshell
 k$ prove t/db_dependent/Koha/Account.t
=> FAIL: The current behaviour doesn't match the described

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>