Bug 27155: Include identifier test in Biblio_and_Items_plugin_hooks.t

Bug 27155: Include identifier test in Biblio_and_Items_plugin_hooks.t

The biblio and item action hooks pass a biblionumber or itemnumber now
to the plugin at time of deletion.
This patch adds a tiny refinement to Biblio_and_Items_plugin_hooks.t
and the associated test plugin to be sure of that.

Test plan:
Run t/db_dependent/Koha/Plugins/Biblio_and_Items_plugin_hooks.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>