They were caused by installing packages with `--ignore-platform-reqs`
which brought PHP 8.1 autoloaded packages and caused fatal errors when
running php-parallel-lint. Instead of ignoring platform requirements we
now remove packages that are incompatible with PHP 7.1 (phpunit and its
dependents).
* Split unit test over multiple jobs in GitHub Actions, lowering the total run time
* rename FileManipulationTest to FileManipulationTestCase so it does not run as a standalone Test