Add Behat tests

Added some behat test covering pickle. Four suites have been created:

* Pickle source (`pickle`)
* Pickle Phar (`pickle_phar`)
* PECL extensions conversion with Pickle source (`pecl`)
* PECL extensions conversion with Pickle Phar (`phar_pecl`)
given that this is a subfolder of the ``pickle`` suite, it already runs as part of the ``pickle`` suite. So you have duplicate tests in your setup