Testing and read session in Model Behavior

I write a behavior for Table. This behavior will check variables in session. Since we can’t access to the request->session() object in behavior, i read session’s variables directly like this :

    private function _getSessionId()
        $session = new Session();
        $organisation_id = $session->read($this->_defaultConfig['session']); // read Auth.User.organisations_id
        return $organisation_id;

this behavior is register to some tables of my application. This session’s reading is call at every before find for model.

Now, i want to test controllers which use tables with this behavior. So i fix var in session like this before the test of the action :

    public function testListeStocksMultiSearch()
        Configure::write('debug', 1);
        // placement de la session
            'Droits' => ['ADMIN'],
            'Auth'   => [
                'User' => [
                    'id'               => 1,
                    'username'         => 'bmarchand',
                    'prenom'           => 'Benjamin',
                    'nom'              => 'Marchand',
                    'organisations_id' => 1
        $url = Router::url(['controller' => 'Stocks',
                            'action'     => 'recherche',
                            '?'          => ['_table'   => 'Stocks',
                                             'Produits' => ['couleur' => 'blanc']]]);

But i always have a null value in the behavior when i try to read the variable Auth.User.organisations_id (it works well when i request the data by the browser).

please someone have an idea about this bug (?).

Many thanks