Hello,
We have enabled parallel runs of PHPUnit extension suite tests for all MediaWiki extensions. This follows after ~3 months of testing on WMDE maintained repositories. We don't anticipate problems, but, if you notice any unusual failures in PHPUnit test suite runs, please file a subtask of T50217: Speed up MediaWiki PHPUnit build by running integration tests in parallel https://phabricator.wikimedia.org/T50217 and add @Arthur_Taylor and @kostajh as subscribers. https://www.mediawiki.org/wiki/Continuous_integration/Tutorials/Debugging_PH... can also be helpful in debugging.
Cheers,
Arthur, Kosta, and Antoine