Fix yet another bug in ON CONFLICT rule deparsing.

Expand testing of rule deparsing a good bit, it's evidently needed.

Author: Peter Geoghegan, Andres Freund
Discussion: CAM3SWZQmXxZhQC32QVEOTYfNXJBJ_Q2SDENL7BV14Cq-zL0FLg@mail.gmail.com