ldb: Prepare to allow tests to operate on ldb_mdb (by using the GUID index)

ldb: Prepare to allow tests to operate on ldb_mdb (by using the GUID index)

The LMDB backend requires the GUID index mode, so prepare for it
by setting a unique objectGUID on each record.  Also prepare for the
index list to be optionally set as an attribute on the test object,
allowing the GUID index mode to be set later when LMDB is configured.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Thu May  3 11:08:12 CEST 2018 on sn-devel-144