[BUG] - Stop loading tests on production environment

[Ferror]

The library loads every class to autoload, which raises two issues.

1. Some of the classes do not comply with PSR-4
2. The test files are loaded on production, which does not make sense

The solution is relatively simple. It would be best if you excluded the Tests namespace from autoload and introduce a new autoload-dev where these are loaded.

Example -> https://github.com/symfony/symfony/blob/6.2/src/Symfony/Bundle/FrameworkBundle/composer.json#L113

Class Payum\Be2Bill\Tests\Be2billOffsiteGatewayFactoryTest located in ./vendor/payum/payum/src/Payum/Be2Bill/Tests/Be2BillOffsiteGatewayFactoryTest.php does not comply with the psr-4 autoloading standard. Skipping.