Bug 14147: Add unit tests to C4::External::OverDrive

Bug 14147: Add unit tests to C4::External::OverDrive

This patch test 'IsOverDriveEnabled' subroutine

    TEST PLAN
    ---------
    1. Apply patch
    2. prove -v t/External/OverDrive.t
    -- All tests should run successfully without
    any error or warning

    TEST PLAN OPTIONAL
    ------------------

    Check with bug 13899 to see the coverage of this module.

    Coverage BEFORE this patch :
    Statement  :  47,5%
    Branch     :   0,0%
    Condition  :   0,0%
    Subroutine :  75,0%

    Coverage AFTER this patch :
    Statement  :  49,1%
    Branch     :   0,0%
    Condition  :  33,3%
    Subroutine :  83,3%

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Tests tun, no koha-qa errors

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>