SQLite in memory problem

Hello, I'm setting CodeIgniter with SQLite in memory DB for unit testing but not working.
This is my configuration:

PHP Code:

$db['test']['hostname'] = '';
$db['test']['username'] = '';
$db['test']['password'] = '';
$db['test']['database'] = ':memory:';
$db['test']['dbdriver'] = 'pdo';
$db['test']['dbprefix'] = '';
$db['test']['pconnect'] = TRUE;
$db['test']['db_debug'] = FALSE;
$db['test']['cache_on'] = FALSE;
$db['test']['cachedir'] = '';
$db['test']['char_set'] = 'utf8';
$db['test']['dbcollat'] = 'utf8_general_ci';
$db['test']['swap_pre'] = '';
$db['test']['autoinit'] = TRUE;
$db['test']['stricton'] = FALSE; 


What I'm doing wrong?

Thanks

PD: Sorry for my english