Bug 32529: Add Koha::Holds->filter_by_found

Bug 32529: Add Koha::Holds->filter_by_found

This patch adds a method for filtering Koha::Holds resultsets by the
status belonging to Koha's concept of 'found'.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Holds.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit f8b7a5a9cd6bbd9ff5f6e58e6e6391178a2588da)
Signed-off-by: Jacob O'Mara <jacob.omara@ptfs-europe.com>