Yii2 basic code coverage added

Added notes about code coverage for `yii2_basic`, since it was fixed in `Codeception` with `Yii2` related issues. 
Will submit for `yii2_advanced` same notes later 

Related with this [one](https://github.com/yiisoft/yii2/issues/4180) and [this](https://github.com/yiisoft/yii2/issues/4692)