CLI integration tests: chgrp plugin

This PR
- adds a first series of integration tests for the CLI `chgrp` plugin
- split the `CLITest` class into two concrete classes to perform CLI tests as root vs non-root
- fix the group existence test as only group names were checked previously
- fixes the plugin to check the session owner is a member of the target group (see https://trac.openmicroscopy.org.uk/ome/ticket/10968)

To test this PR:
- check the integration tests pass in the OmeroPy-integration-develop job
- check all the following fail gracefully with a comprehensible message:
 ````
bin/omero chgrp -1 /Image:id
bin/omero chgrp Group:-1 /Image:id
bin/omero chgrp ExperimenterGroup:-1 /Image:id
bin/omero chgrp this_is_not_a_group /Image:id
````

- check
 ```
bin/omero chgrp gid /Image:id
```

 fails if the session owner is not a member of `gid`, whether he is an admin or not.

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--no-rebase