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

src/test/perl/PostgresNode.pm

@@ -1938,7 +1938,7 @@ sub connect_fails
 	# have set up things properly, and this should not block.
 	my ($ret, $stdout, $stderr) = $self->psql(
 		'postgres',
-		"SELECT \$\$connected with $connstr\$\$",
+		undef,
 		extra_params => ['-w'],
 		connstr      => "$connstr");