Maybe some of those tests could be written via PHPUnit? I don't know how to simulate two MediaWiki instances in there though :-\

I have long wanted that, and I have recently worked on making this possible as a personal side project. However, the devil is in the details...

Have a look at https://phabricator.wikimedia.org/T261848 if you are interested.

-- 
Daniel Kinzler
Principal Software Engineer, Core Platform
Wikimedia Foundation