Add integration tests for the CLI user plugin

Similar to #1771, this PR adds integration tests for the `bin/omero group` plugin. Tests expected to fail due to https://trac.openmicroscopy.org.uk/ome/ticket/11687 are marked with `py.mark.xfail`.

To test this PR, check the output of [OmeroPy-integration-develop](http://hudson.openmicroscopy.org.uk/job/OmeroPy-integration-develop/)

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