10618 priv thumbs (rebased onto develop)



This is the same as gh-1500 but rebased onto develop.

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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.