migrate mediawiki-core-phpunit-* to slaves

Makes some mw phpunit jobs to run on slaves instead of master. That is
made possible after some tweaks to the gallium installation.

Updated jobs:

mediawiki-core-phpunit-api
mediawiki-core-phpunit-databaseless
mediawiki-core-phpunit-misc
mediawiki-core-phpunit-parser

bug: 50228
Change-Id: Ie8e9cfe6cfc03a503451583b3e1207c40a5b6358