Add integration tests for the CLI group plugin (help and add subcommand)

- Test help for all subcommands as non-root
- Test all arguments of omero group add subcommand as root
Test defaultperms dictionary defined in group plugin
Add matrix tests for group perms subcommand
Parametrize perms key/value pairs as tuple using py.mark.parametrize
Pass group parameters as prefix/attribute pairs
Add integration tests for group adduser subcommand
Add integration tests for group removeuser subcommand
Add integration tests for copyusers subcommand
Add integration tests for group list subcommand
Simplify testList integration test
Decorate failing tests with pytest.mark.xfail decorator
Remove py.mark.xfail decorators (see #11687)
Merge pull request #1771 from sbesson/clitest_group

Add integration tests for the CLI group plugin