Bug 27995: Unit tests

Bug 27995: Unit tests

This patch updates the unit tests for Koha::Account::Line->apply to
reflect the change we wish to make to the return value.

Instead of returning the scalar amount of available credit left on the
credit line after calling apply, we want to return the full, updated,
Koha::Account::Line object including Koha::Object::Messages for the
result of renewals that may have taken place.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>