Lumen cache default driver issue

I want to know how lumen set default cache driver. I know that it has a cache.php in the vendor/laravel and I should set CACHE_DRIVER in my .env file.

But it didn't work.

class ExampleTest extends TestCase
{
    /**
     * A basic test example.
     *
     * @return void
     */
    public function testExample()
    {
        Cache::put('testkey',456,60);
        $this->assertEquals(456,Cache::get('testkey'));
    //This var_dump output is : ''array"
        var_dump($this->app['cache']->getDefaultDriver());
        $this->get('/');

        $this->assertEquals(
            $this->app->version(), $this->response->getContent()
        );
    }
}