Bug 21727: Unit tests for the refund case

Bug 21727: Unit tests for the refund case

This patch introduces tests for the behaviour when the negative adjust
exceedes the amount that was already paid. In this case, the line amount
is expected to be set to 0, and a credit with the payed amount created.

To test:
- Apply this patch
- Run:
  $ kshell
 k$ prove t/db_dependent/Koha/Account/Lines.t
=> SUCCESS: tests pass!

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