Bug 24185: Make holds page faster: Preparatory refactoring

Bug 24185: Make holds page faster: Preparatory refactoring

This is just refactoring. extracting logically independent code
to separate sub + tests update. No logic change yet.

Searching for "any_available" item among all biblionumber items was done
inside of "elsif on_shelf_holds == 2", and it is logically very independent
piece of code (this "@items" loop), it needs just biblionumber and patron
as parameters so it can be extracted into separate subroutine, and
later also called/reused from somewhere else.

This ability to call from another place also made for future patch
to remove O(n^2) problem with nested loops.

Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>