test.php wastes CPU cycles and bandwith downloading PHPUnit every time

I noticed that running php test.php downloads PHPUnit on every run, even if phpunit is already in the system and even if phpunit.phar has already been downloaded.

$ phpunit --version
PHPUnit 5.7.27 by Sebastian Bergmann and contributors.

$ ll -h /tmp/wp-test-runner/phpunit.phar 
-rw-rw-r-- 1 benchmark benchmark 2.9M Feb  1 07:56 /tmp/wp-test-runner/phpunit.phar

$ php test.php
Environment variables pass checks.
cd '/tmp/wp-test-runner'; php phpunit.phar

PHP version: 7.0.30-1+ubuntu14.04.1+deb.sury.org+1 (/usr/bin/php7.0)

Installing...
Running as single site... To run multisite, use -c tests/phpunit/multisite.xml
Not running ajax tests. To execute these, use --group ajax.
Not running ms-files tests. To execute these, use --group ms-files.
Not running external-http tests. To execute these, use --group external-http.
PHPUnit 5.7.27 by Sebastian Bergmann and contributors.

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....

Start in test.php and inspect how it locates or downloads PHPUnit. Run php test.php with PHPUnit available as phpunit and with phpunit.phar already present, then verify the existing installation or archive is reused instead of downloaded again.