Added test-overview

docs/guide/test-overview.md

 Testing
 =======
 
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+Testing is an important part of software development. Whether we are aware of it or not, we conduct testing continuously.
+For example, when we write a class in PHP, we may debug it step by step or simply use echo or die statements to verify
+that implementation is correct. In case of web application we're entering some test data in forms to ensure the page
+interacts with us as expected. The testing process could be automated so that each time when we need to test something,
+we just need to call up the code that perform testing for us. This is known as automated testing, which is the main topic
+of testing chapters.
+
+The testing support provided by Yii includes:
+
+- [Unit testing](test-unit.md) - verifies that a single unit of code is working as expected.
+- [Functional testing](test-functional.md) - verifies scenarios from a user's perspective via browser emulation.
+- [Acceptance testing](test-acceptance.md) - verifies scenarios from a user's perspective in a browser.
+
+
+Test environment setup
+----------------------
+
+The unit testing supported provided by Yii requires [PHPUnit](http://phpunit.de/). In order to run functional and
+acceptance tests you need [Codeception](http://codeception.com/).
+
+### Installing PHPUnit
+
+It's best to install PHPUnit globally. Since we have Composer we can do it with a single command:
+
+```
+composer global require "phpunit/phpunit=4.1.*"
+```
+
+After running the command you'll see "Changed current directory to /your/global/composer/dir" message. If it's the
+first time you're installing a package globally you need to add `/your/global/composer/dir/vendor/bin/` to your `PATH`.
+
+Now we're able to use `phpunit` from command line.
+
+Check [PHPUnit documentation](http://phpunit.de/manual/current/en/installation.html) for more ways of installing it.
+
+### Installing Codeception
+
+Codeception can be installed globally the same way as PHPUnit:
+
+```
+composer global require "codeception/codeception=2.0.*"
+composer global require "codeception/specify=*"
+composer global require "codeception/verify=*"
+```
+
+Now we're able to use `codecept` from command line.