Do not depend on current working directory to find config.php in tests

Unit tests for [NetgenTagsBundle](https://github.com/netgen/TagsBundle) can be ran from the root directory of eZ Publish 5. They require that the file `vendor/ezsystems/ezpublish-kernel/config.php-DEVELOPMENT` is copied to `vendor/ezsystems/ezpublish-kernel/config.php` and since `vendor/ezsystems/ezpublish-kernel/` directory is not the current working directory, some of the tests fail.

This change makes sure that `config.php` is found by using the `__DIR__` constant rather than depending on current working directory.