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