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