As part of a staged implementation of native PHP parallel PHPUnit testing, and following on from T361190, create a composer command to generate split groups of tests from a specified (or the default) test suite.
Splitting the suite into smaller groups will allow us to run those groups locally or from Quibble by passing the --testsuite argument to PHPUnit.
When this is implemented as a composer task in Mediawiki, we will be able to remove the Python splitting implementation introduced into Quibble in T361190.
Acceptance Criteria
- Running composer phpunit:prepare-parallel-test-suites generates a phpunit.xml containing split groups of tests
- Executing each of the newly generated test suites individually runs the same set of tests that running the original suite does.