Bug 30663: Add Koha::Suggestions helper methods

Bug 30663: Add Koha::Suggestions helper methods

This patch adds the following helper methods:

* filter_by_pending
* filter_by_suggested_days_range

This methods follow basically what's done in opac-suggestions.pl
I chose 'pending' as opposed to 'open' to follow what we use in the UI
which might be the case because of being more accurate for end users.

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

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>