Completed the needed context to successfully test commands with Helpers

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4581 

As mentioned in symfony/symfony#12798 the explanation about the initialization is not clear.
Therefore I have added the needed context and a reference to the related cookbook article.
By the way, we have a section on testing commands in the components docs: http://symfony.com/doc/current/components/console/introduction.html#testing-commands

So, probably it's best to link there instead to the cookbook.