script: Add new (temporary) pure python3 ad-dc-2 test

script: Add new (temporary) pure python3 ad-dc-2 test

Ideally we want all the tests to run under python3 by default (no
special task for this) and then convert the existing '-py3' tasks
to run the python tests with python3.
However at the moment the convertion process is not ready to do this,
for a while we need to run separate autobuild tasks for this.

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>