Add some missing Brazilian Portuguese translations to Yii Definitive Guide

I've translated:

* docs/guide/pt_br/extension.create.txt
* docs/guide/pt_br/extension.integration.txt
* docs/guide/pt_br/upgrade.txt
* docs/guide/pt_br/test.functional.txt
* docs/guide/pt_br/test.unit.txt
* docs/guide/pt_br/test.fixture.txt
* docs/guide/pt_br/test.overview.txt

I hope to finish remaining translations until sunday.