mw-run-phpunit*: let us override PHPUNIT_DIR

I would like to run MediaWiki PHPUnit job whenever a change is proposed
to our PHPUnit fork in integration/phpunit.git.  To do so change the
assignement of PHPUNIT_DIR so it does not override the variable if it is
already set.

Change-Id: I26ff7a52b9ab9fefef5dfb4c07ee65738e3f9868