Bug 19036: (follow-up) Test credits should be credits

Bug 19036: (follow-up) Test credits should be credits

A series of accountlines added to mock credits were being added with a
debit_type_code instead of a valid credit_type_code and so were being
mis-identified by the new credit numbering code trigger in the store
routine.

This patch updates the test to correctly identify the credits as credits

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>