Bug 22440: Add Koha::Illrequests->filter_by_visible

Bug 22440: Add Koha::Illrequests->filter_by_visible

This patch introduces a method for filtering out requests that match the
statuses specified on the *ILLHiddenRequestStatuses* system preference.

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

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 1a186826e46212ed7a2acbf9e63e0b6eb8e6cc79)
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>