Add mdb_rls regression test for mdb_superuser RLS interaction

[reshke]

mdb_superuser bypasses RLS via object_ownercheck() in check_enable_rls(), which uses has_privs_of_role() to determine ownership. This test verifies:

1. A plain role sees only RLS-filtered rows.
2. mdb_superuser bypasses RLS entirely (treated as table owner).
3. With FORCE ROW LEVEL SECURITY, mdb_superuser is subject to RLS but still matches any policy via the rowsecurity.c bypass.