Laravel testing with group prefix

I have a group for api:

Route::group(array('prefix' => 'api'), function () {

    Route::get('abc', function () {
        return 'abc';
    });
  );

But, when testing use $this->call('GET', '/api/abc) or $this->call('GET', 'path_to_abc'), I always get 404 error, although I've printed the url and I can visit that link on browser

via Chebli Mohamed