Tests improved

Added permanent user logout to avoid different data-collisions. 
Currently per each `login()` or `...session->open()` new session will be started since there is no session id that can be passed from cookies in console, it is ok.

@qiangxue do we need to do some cleanup in `yii\codeception\TestCase` to avoid data-collisions since they all run in one thread in console, or it is fine to mention later in docs so that they can use `@backupGlobals` phpunit [annotation](http://phpunit.de/manual/3.7/en/appendixes.annotations.html) in test? 
However maybe some component should be cleaned - runtime dir, cache, etc?