Bug 31051: (follow-up) Tests for get_savings and more

Bug 31051: (follow-up) Tests for get_savings and more

- Added tests in t/db_dependent/Koha/Patron.t
- Added wording to OPACShowSavings syspref about anonymised checkout
history
- Added IDs to the savings messages on the OPAC
- Prevent explosion if a checked out item has been deleted

Sponsored-by: Horowhenua Libraries Trust
Signed-off-by: Hammat Wele <hammat.wele@inlibro.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>