Find a way to avoid composer update when running tests with several PHP versions

[jclaveau]

Currently a composer update is required when we change the PHP version we use, due to the changes of dependencies.

• Using one vendor directory by PHP version would avoid this issue https://getcomposer.org/doc/03-cli.md#composer-vendor-dir
• Use a specific composer.lock for each PHP version https://getcomposer.org/doc/03-cli.md#composer
• Create à ./composer file that will use the right composer.json file (symlink or copy)
• Do not specify this option in composer.json as this is only required for --dev environment