python/samba/tests: make sure samba.tests can be imported without SamDB

python/samba/tests: make sure samba.tests can be imported without SamDB

We are using samba.tests Python module __init__.py file as a catch-all
for all types of helpers. Some of these helpers are only usable with
Samba AD DC targets.

When SamDB is not available in a non-Samba AD DC target, provide a
dummy replacement that simply returns None. This allows to complete
initialization for non-Samba AD DC target tests which do not use
connect_samdb() helper.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13542

Signed-off-by: Alexander Bokovoy <ab@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>
Autobuild-User(master): Günther Deschner <gd@samba.org>
Autobuild-Date(master): Mon Jul 23 19:36:56 CEST 2018 on sn-devel-144