Install PHPUnit as PHAR

Summary:
Packages can be out of date for PHPUnit.

Furthermore, this allows to use more easily the run-php-script
wrapper, so any version installed through Composer will be used
instead of the system one.

Test Plan:
Tested on WindRiver

phpunit --version

Reviewers: DorianWinty

Reviewed By: DorianWinty

Differential Revision: https://devcentral.nasqueron.org/D2808