Bug 25508: Add Unit Tests

Bug 25508: Add Unit Tests

Enhance the unit tests to check for the return values of
Koha::Account->pay.

Test plan
1/ Apply just this patch
2/ Run t/db_dependent/Koha/Account.t and watch it fail
3/ Apply the rest of the patch series
4/ Run t/db_dependent/Koha/Account.t and watch it pass
5/ Signoff

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>