selftest: fix creation of builtin users in wait_for_start

selftest: fix creation of builtin users in wait_for_start

If "BUILTIN\Users" already exists, attempting to create it would fail,
so we should check for the existence prior to the creation.

It is unclear *why* the mapping sometimes already exist and sometime
not. There are two places where they would have been created:

1. libnet_join_add_dom_rids_to_builtins tries to add the mapping when
joining a domain, but at that point winbindd isn't running

2. when a user is authenticated in smbd, which clearly can't have
happended when in the function wait_for_start

Go figure...

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>