Switch pg_basebackup commands in Postgres.pm to use --nosync

On slow machines, this greatly reduces the I/O pressure induced by the
tests.

From: Michael Paquier <michael.paquier@gmail.com>

src/test/perl/PostgresNode.pm

@@ -483,7 +483,7 @@ sub backup
 
 	print "# Taking pg_basebackup $backup_name from node \"$name\"\n";
 	TestLib::system_or_bail('pg_basebackup', '-D', $backup_path, '-p', $port,
-		'-x');
+		'-x', '--nosync');
 	print "# Backup finished\n";
 }