Bug 24295: Replace mock in t/db_dependent/Circulation/transferbook.t

Bug 24295: Replace mock in t/db_dependent/Circulation/transferbook.t

Replace the call to GetTransfers with the get_transfer method in
Koha::Item in transferbook.t

Test plan
1/ Run t/db_dependent/Circulation/transferbook.t
2/ Confirm it passes
3/ Apply patch
4/ Repeat steps 1-2

Rebased-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>