Add missing BUFFERS OFF in select_into regression tests
c2a4078eb adjusted EXPLAIN ANALYZE to include BUFFERS by default, but
a few tests in select_into.sql neglected to add BUFFERS OFF. The
failing tests seem unlikely to ever access buffers during execution, but
they certainly could during planning.
Per buildfarm member kestrel, tayra and calliphoridae.
Discussion: https://postgr.es/m/CANNMO++W7MM8T0KyXN3ZheXXt-uLVM3aEtZd+WNfZ=obxffUiA@mail.gmail.com