Bug 27071: More tests for Koha::Library->validate_hold_sibling

Bug 27071: More tests for Koha::Library->validate_hold_sibling

This patch adds tests for more than one non-same sibling libraries in
the hold group. Tests pass which means it is working as expected.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Libraries.t
=> SUCCESS: Tests pass

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>