[BUGFIX] Ensure TestCommand::execute() interface compatibility

[moritz-ngo]

This PR fixes the following issue:

Fatal error: Declaration of
StudioMitte\Brevo\Command\TestCommand::execute(Symfony\Component\Console\Input\InputInterface $input, Symfony\Component\Console\Output\OutputInterface $output)
must be compatible with 
Symfony\Component\Console\Command\Command::execute(Symfony\Component\Console\Input\InputInterface $input, Symfony\Component\Console\Output\OutputInterface $output): int
in /var/www/html/vendor/studiomitte/brevo/Classes/Command/TestCommand.php on line 39