[2.2] add explicit profiler enabling, necessary on 2.2

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.2+
| Fixed tickets | None

In Symfony 2.2 you have to explicitly enable profiler in functional testing to inspect collected data.

This is covered in http://symfony.com/doc/current/cookbook/testing/profiling.html but I thought it would be better to have it documented also in this cookbook