ipatests: Add tests for `schema` Command
    
    - the base testing of this command is made by ipaclient `schema`
    remote plugin, but some specifics are not covered
    
    - allow testing of the plugin in `development` mode(locked API).
    
    Fixes: https://pagure.io/freeipa/issue/8955
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>