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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