Use fast checkpoint in PostgresNode::backup()

Should cause tests to be a bit faster

src/test/perl/PostgresNode.pm

@@ -553,8 +553,10 @@ sub backup
 	my $name        = $self->name;
 
 	print "# Taking pg_basebackup $backup_name from node \"$name\"\n";
-	TestLib::system_or_bail('pg_basebackup', '-D', $backup_path, '-h',
-		$self->host, '-p', $self->port, '--no-sync');
+	TestLib::system_or_bail(
+		'pg_basebackup', '-D', $backup_path, '-h',
+		$self->host,     '-p', $self->port,  '--checkpoint',
+		'fast',          '--no-sync');
 	print "# Backup finished\n";
 	return;
 }