Move eZScript instance into ezpTestSuite, simplifies runtests.php

This PR slims down eZ Publish dependencies in the `runtests.php` script to a minimum and moves the `eZScript` initialization into the base `ezpTestSuite`.

This puts the `eZScript` instance where it belongs (inside the Suite, not in the global scope) and is a preparation to get rid of the `runtests.php` in the long term.

Cheers
:octocat: Jérôme