Add integration tests for the CLI group plugin

- Add integration tests for all the subcommand of `bin/omero group`
- Use parametrisation to try out all the combinations of optional arguments

To test this PR, check the corresponding test results in [OmeroPy-integration-develop](http://hudson.openmicroscopy.org.uk/job/OmeroPy-integration-develop/) are green.

NB:  all the `bin/omero group removeuser`  tests running without the `--as-owner` option are currently expected to fail due to a bug in the `AdminImpl`. See https://trac.openmicroscopy.org.uk/ome/ticket/11687. /cc @mtbc

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--no-rebase