Bug 19915: (QA follow-up) Tidy up GetItemsForInventory.t

Bug 19915: (QA follow-up) Tidy up GetItemsForInventory.t

Standard edits like modules, schema, Koha objects instead of DBIC.
Removing unneeded autoflush.
Moving initial test to a first subtest.
No need to clear issues if you clear items.
Adding a FIXME for OldWay; would be nice to remove that code..

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
https://bugs.koha-community.org/show_bug.cgi?id=19905

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>