Bug 26405: Prevent Circulation.t failing on 'AddRenewal left both fines'

Bug 26405: Prevent Circulation.t failing on 'AddRenewal left both fines'

Bug 23051 added option to skip calculating fines even when
"CalculateFinesOnReturn" is enabled. Due this Circulation.t
fails on  test  "AddRenewal left both fines" if
"CalculateFinesOnReturn"  is disabled on ones system.
Mocking syspref while testing fixes failig tests.

To test:
1. Disable "CalculateFinesOnReturn".
2. Run prove t/db_dependent/Circulation.t
=> Tests fail
3. Apply patch.
4. Run prove t/db_dependent/Circulation.t again
=> Tests are succesful

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>