Suffix mediawiki-phpunit with '-zend' + enable hhvm

Rename:
- mediawiki-phpunit
+ mediawiki-phpunit-zend

Add to test/gate-and-pipeline the job mediawiki-phpunit-hhvm as a non
voting job pending resolution of T75531.

Remove hhvm job from experimental pipeline.

Bug: T75521
Change-Id: Ib3135a739e7f628aac561158351f3227c43cd1b2