CPP unit test fixes

While testing @rleigh-dundee's work on breaking the `omero_client` library into multiple smaller libraries, I spent some time trying to get the unit tests passing (mostly around the comparison operators). Still outstanding is the suggestion here: https://github.com/joshmoore/openmicroscopy/commit/db1e494912262e37a5c4bbef29740dbcf0a77477#commitcomment-6210259