[enh] parallel sessions with `$I->haveFriend()`

[mytskine]

Functional and acceptance tests have a procedure for handling simultaneous sessions in a single test with Multi Session Testing. Currently this fails with the Yii2 functional tests, with the error message:

[TestRuntimeException] No multisession modules used. Can't instantiate friend

Codeception Yii2 module should implement MultiSession. Pull request follows.