GH Actions: fix tests not running on PHP 8.1
GH Actions: fix tests not running on PHP 8.1

The tests weren't running on PHP 8.1 because PHPUnit 5.x was being installed as not all PHPUnit dependencies have declared PHP 8.1 compatibility yet.

Fixed by running `composer install` with the `--ignore-platform-requirements` option for now.

In the semi-near future, it should be possible to remove the special casing of PHP 8.1.

antecedent/patchwork