tests dsdb: Add tests for optionally unique objectSID's

tests dsdb: Add tests for optionally unique objectSID's

It is possible for foreign security principals to have duplicate object
sids, this can be the result of:
 a replication race condition generating conflict resolution objects
 or the foreign security principal being deleted and then re-added on a
 join.

Rather than remove unique check on all objectSIDs we wish to allow
duplicate objectSIDs for foreign security principals.  But enforce the
unique constraint for local objects.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13004

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>