Consolidate testing environment

- Remove `tests/bootstrap.php` by:
  * Using `autoload-dev` in Composer instead of
    manually adding namespaces to its autoloader on the fly
  * Using PHPUnit `<ini>` XML configuration to set up
    `display_errors`, `error_reporting` and `date.timezone`
- Require `phpunit/phpunit` as a dev dependency instead of depending
  on the platform installation.
  See discussion about that here: https://github.com/silexphp/Silex/pull/1075
- Use new Composer custom commands with `scripts`.
  `composer run-script test` is now just `composer test`.

So you could now just `composer install` and `composer test`. Everything
else is taked care of in a minimalistic, but still well-defined way.