Change PostgresNode::connect_fails() to never send down queries

 Change PostgresNode::connect_fails() to never send down queries
 
 This type of failure is similar to what has been fixed in c757a3da,
 where an authentication failure combined with psql pushing a command
 down its communication pipe causes a test failure.  This routine is
 designed to fail, so sending a query has little sense anyway.
 
 Per buildfarm members gaur and hoverfly, based on an analysis and fix
 from Tom Lane.
 
 Discussion: https://postgr.es/m/513200.1617634642@sss.pgh.pa.us