Corrected call_user_func arguments in Yii2

[iRipVanWinkle]

This code

// inside Cest file or Codeception\TestCase\Unit
public function _fixtures()
{
    return ['posts' => PostsFixture::className()]
}

threw exception: "call_user_func() expects parameter 1 to be a valid callback, no array or string given"

and I fixed it up

[DavertMik]

Thank you!