Bug 28855: Purging suggestions test should not be on timestamp

Bug 28855: Purging suggestions test should not be on timestamp

DelSuggestionsOlderThan is on "date", a timestamp. It can be a problem
because "date" is updated when we update the database, in consequences
all the suggestions take the same date.

This patch use 'manageddate' instead.

In order to test this patch, you will need to have some suggestions
where "Managed on" (suggestions.manageddate in the database) is older
from "Last updated" (suggestions.date in the database). This can append
during some specific update. If you don't have such data, you will have
to add them via SQL request.

Example of sql for insertion (ID is the patron id of a librarian which
can manage suggestions):
INSERT INTO suggestions (suggestedby, suggesteddate, managedby,
manageddate, acceptedby, accepteddate, STATUS, title) VALUES (1,
'2020-10-13', 1, '2020-10-26', 1, '2020-10-26', 'ACCEPTED', 'title');

Test plan:
1- Choose a date and find (or add) a suggestion where:
    - "Last updated" is not older than this date
    - "Managed on" is older than this date
    - "Status" is different from "Pending" ("ASKED" in database)
Calculate number_days = today's date - chosen date
Example: (Today's date 28/10/2021, chosen date : 28/10/2020,
number_days=365)
Suggestion | .. | Managed on | .. | Last updated | .. | Status
title      | .. | 10/26/2020 | .. | 05/15/2021   | .. | Accepted

2- Run cron job
/home/koha/src/misc/cronjobs/purge_suggestions.pl --confirm --days number_days

3- Check that this suggestion has not been deleted

4- Apply the patch

5- Repeat steps 1 and 2

6- Check that this suggestion has been deleted

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>