In support of testing more powerful / faster CI configurations (T383254), make it possible to configure the split group count via an environment variable.
The current setting (8 groups) is hard-coded in core/includes/composer/ComposerLaunchParallel.php:
private const SPLIT_GROUP_COUNT = 8;
It should be as simple as processing an environment variable here (QUIBBLE_PHPUNIT_PARALLEL_GROUP_COUNT ?), and ensuring there is a default value of 8 (and that the value is never less than 2).