[Debug] Fix tests on PHP 5.6

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

On PHP 5.6 [debug tests are failing](https://travis-ci.org/symfony/symfony/jobs/29118115#L357) with:

```
PHP Fatal error:  Class 'Psr\Log\LogLevel' not found in /home/travis/build/symfony/symfony/src/Symfony/Component/Debug/ErrorHandler.php on line 522
```

I think https://github.com/symfony/Debug/commit/4bd5484d456e6263acc6429fabea7e2fbc74fffc#diff-b91ca53d6ab50b804cb5783aaf1f9740R71 introduced the problem. Alternative fix would be to initialize the `$loggers` in a constructor.

ping @nicolas-grekas