Bug 27797: (QA follow-up) Pickup locations can be overridden too

Bug 27797: (QA follow-up) Pickup locations can be overridden too

This patch adds regression tests for overridding pickup locations, which
was inadvertedly not covered by the original dev. What this does, is
moving the $can_override variable definition above, and avoid returning
400 if conditions are not met for the passed pickup_library_id, when
$can_override is true.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/holds.t
=> SUCCESS: Tests pass!

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>