Make PostgresNode.pm's poll_query_until() more chatty about failures.

Reporting only the stderr is unhelpful when the problem is that the
server output we're getting doesn't match what was expected.  So we
should report the query output too; and just for good measure, let's
print the query we used and the output we expected.

Back-patch to 9.5 where poll_query_until was introduced.

Discussion: https://postgr.es/m/17913.1539634756@sss.pgh.pa.us

src/test/perl/PostgresNode.pm

@@ -1239,9 +1239,18 @@ sub poll_query_until
 		$attempts++;
 	}
 
-	# The query result didn't change in 180 seconds. Give up. Print the stderr
-	# from the last attempt, hopefully that's useful for debugging.
-	diag $stderr;
+	# The query result didn't change in 180 seconds. Give up. Print the
+	# output from the last attempt, hopefully that's useful for debugging.
+	chomp($stderr);
+	$stderr =~ s/\r//g if $TestLib::windows_os;
+	diag qq(poll_query_until timed out executing this query:
+$query
+expecting this output:
+$expected
+last actual query output:
+$stdout
+with stderr:
+$stderr);
 	return 0;
 }