In T361190, we look at splitting the PHPUnit extensions test suite into smaller parts to make them run faster. The implementation there is as additional commands for Quibble, which makes it possible to run the suite in parallel in CI.

For developers working on Mediawiki locally, it would also be a big support to be able to run the tests in parallel locally. Making local reproduction of parallel test runs easier for developers would also support faster debugging of issues that show up in parallel test runs in CI.

Port the Python test suite splitting implementation to PHP (or use the existing PHP implementation in phpunit-quibble-splittool, and create a new composer task that runs the given suite in parallel.

Acceptance Criteria

• Mediawiki has a new composer phpunit:parallel entrypoint that runs tests in parallel
• The same set of tests run from the composer phpunit:parallel entrypoint that run from the normal composer phpunit:entrypoint command
• It is possible for developers to run the individual split groups as serial test suites to debug issues that show up in parallel execution
• The parallel functionality is documented