deployment: integration/phpunit for Jenkins CI slaves

This would let us deploy a snapshot of PHPUnit from a Wikimedia git
repository which has PHPUnit installed via composer.  That:

- make sure we do not install from a third party (aka pear/packagist)
- workaround the lack of PHPUnit 3.7.x package in Debian/Ubuntu.

The documentation to update PHPUnit is in the integration/phpunit.git
repository.

bug: 54762
Change-Id: Ic4316884cfb2b2c6aa615b29a9a71d502d74e3b4
Merge "deployment: integration/phpunit for Jenkins CI slaves" into production