ipatests: Add tests for `plugins` plugin
    
    Previously there were no tests for `ipalib.misc` module.
    
    Fixes: https://pagure.io/freeipa/issue/8898
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>