To allow installing PHPUnit versions that successfully run on PHP 7.4 (see PHPLIB-462), we use Symfony's PHPUnit bridge to provide a compatibility layer for older PHPUnit releases. This compatibility layer will be released with Symfony 4.4 in November 2019. Until the release, we temporarily rely on a development version of Symfony to run our unit tests.

After the release of Symfony, this requirement in composer.json needs to be changed to target a stable Symfony release.