Allow use of phpunit.phar with MediaWiki's phpunit.php test entry point

MediaWiki tends to assume installation of phpunit via PEAR.
Distribution of phpunit via PEAR will eventually be discontinued,
in favor of distribution via PHAR or composer. PEAR installation
of phpunit 4.0 essentially distributes a phar.

https://github.com/sebastianbergmann/phpunit/wiki/Release-Announcement-for-PHPUnit-4.0.0

This patch allows phpunit.php to work with a phpunit.phar file,
as long as it's in the include path.  --with-phpunitdir can be used
to add a directory to the include path.

With composer install, no options are needed and it works.

Bug: 58881
Change-Id: Ibac9b36bbb9614d990c697a091947a64c8987d1d