Testing docs overview

Writed some docs about testing, basing on Yii1 guide and some other things from books. Recommended book of Kent Beck as it contains good examples and have a lot of answers for different TDD questions for newly developers.
Separated testing environment in particular guide, added two guides for applications templates, they are TBD, when this [issue](https://github.com/yiisoft/yii2/issues/4767) will be solved.