test_batch_plugin: fix py2/3 failing assertion
When running "test_batch_plugin" with Py2 against Py3 server we
got assertion error due to a command trying to run as bytes.
E.g.: unknown command 'b'ping''
https://pagure.io/freeipa/issue/7131
Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>