s3/script/test: Use different testdir for samba3.blackbox.smbclient_tarmode

s3/script/test: Use different testdir for samba3.blackbox.smbclient_tarmode

The other tarmode torture test samba3.blackbox.smbclient_tar now uses a share
'tarmode' which uses the same source path as samba3.blackbox.smbclient_tarmode

Avoid conflicting paths and use a new subdir (of the test share) called
'smbclient_tarmode'

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14581

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Jeremy Allison <jra@samba.org>