10618 priv thumbs

Test enhancements for investigating https://trac.openmicroscopy.org.uk/ome/ticket/10618#comment:7

All the tests should pass (excluding the one that is marked "xfail"). None of the tests should a problem of the form described in 10618, but it's clear that proper use of "omero.group" is required when interacting with the thumbnail store. gh-1441 makes changes on `develop` which may be related, but this PR will need to be rebased to properly test that.