selftest:Samba3: provision the domain adminstrators group in the s3 environments

selftest:Samba3: provision the domain adminstrators group in the s3 environments

I discovered that this sid / mapping is missing by working with the Sids2Uids
code and test. I do even wonder why this test could succeed prior to my pending
changes to the winbindd sids-to-xids code, for example against the s3:local
environment, since the test tries to map the sid <domsid>-512.

Signed-off-by: Michael Adam <obnox@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
(cherry picked from commit ee17a516c82acbdf347c2a47e7003b6a7fb879de)