Bug 16134: t::lib::Mocks::mock_preference should be case-insensitive

Bug 16134: t::lib::Mocks::mock_preference should be case-insensitive

Test plan:
1. prove t/db_dependent/Auth_with_cas.t => FAIL
2. Apply patch
3. prove t/db_dependent/Auth_with_cas.t => SUCCESS

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>
(cherry picked from commit 300bf096bda90a0f17c6615798957fa7f588b36e)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>