s3:tests: Fix authentication with smbget_user in smbget tests

s3:tests: Fix authentication with smbget_user in smbget tests

Currently the smget share is broken. We set `guest ok = yes` so if you
specify invalid names, the authentication will still succeed as we
are mapped to guest.

The smbget_user is a local ad_member user. We need to set the
workstation as the "domain" for the user.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15532

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
(cherry picked from commit c14c5dec09fe1c86b29b3091ad521e73a2e1c3e9)