When installing all the MediaWiki extensions currently deployed on the Wikimedia cluster with a dummy LocalSettings.php, we end up with a lot of failures, bad interactions between extensions or core tests broken via hooks. Some extensions even end up throwing fatal errors.
To help migrate to HHVM, we would need to be able to run the unit tests of core and of all the extensions and have them pass.
Ori has worked on this a bit for HHVM and already reported a fair amount of bugs.
Antoine has been working on an experimental Jenkins Job that run all wmf extensions tests.
Lets join the effort, fill bug and track them there. Then reach out to the other developers and get everything fixed.