Bug 30234: Fix local cover image display for other holdings

Bug 30234: Fix local cover image display for other holdings

Bug 26145 added the ability to add local cover images to items.
If SeparateHoldings is set, items from other libraries are displayed in
a separate tab, and not considered as visible. We have a JS test to
assume that images not shown are not present, this only apply for the
images for the bibliographic record, as they can come from different
sources.

Test plan:
Turn on LocalCoverImages and SeparateHoldings
Create a record with different items coming from different libraries
Attach images to items and to the biblio
Notice that with this patch you can see the images from the "Other
holdings" tab

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>