Bug 19307: Mock the AllowFineOverride preference to ensure expected result

Bug 19307: Mock the AllowFineOverride preference to ensure expected result

To test:
 1 - Set 'AllowFineOverride' to allow
 2 - prove t/db_dependent/Circulation/NoIssuesChargeGuarantees.t
 3 - 1 test fails
 4 - Apply patch
 5 - prove t/db_dependent/Circulation/NoIssuesChargeGuarantees.t
 6 - All tests pass
 7 - Set 'AllowFineOverride' to 'Don't allow'
 8 - Tests should still pass

Signed-off-by: David Bourgault <david.bourgault@inlibro.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>