Automagically include the latest version of phpunit.phar in the repo

[akoebbe]

Rather than manually checking in the latest phpunit.phar file and rename it to contemplate_koans, we should make it an automatic download/checksum/chmod/rename via CI (github actions) whenever new code is pushed in to php_koans. This will also protect against a malicious version of phpunit sneaking via a contributor. I haven't messed with Github actions to know if this is possible or straightforward.