[DEPRECATION] comply with psr-4 autoloading standard

[florianajir]

Deprecation warning on composer install:

Deprecation Notice: Class Bazinga\Bundle\JsTranslationBundle\Tests\AppKernel located in ./vendor/willdurand/js-translation-bundle/Tests/Fixtures/app/AppKernel.php does not comply with psr-4 autoloading standard. It will not autoload anymore in Composer v2.0.

[stof]

Several solutions here:

• restructure the bundle to move tests and fixtures outside of the non-dev autoloading root
• use exclude-from-classmap to exclude the Tests folder from the optimized autoloader
• use .gitattributes to exclude the Tests folder from the download (this will only remove the warning for dist installs, not source installs).