Something like the phpunit/paratest wrapper at https://github.com/AaronSchulz/mediawiki-developer-tools would be nice to have in MediaWiki core.
- Merge mediawiki-developer-tools suite.xml optimizations (or an improved version) into core (these breaks up the tests into more suites that can be run in parallel)
- Add support for cloning to new sqlite DB files in CloneDatabase; this will replace wfCloneSqliteSchemaForParatest() from mediawiki-developer-tools
- Add paratest to composer.json
- Merge ParatestWrapperSettings.php into core from mediawiki-developer-tools
- Include a simplified version of mediawiki-phpunit from mediawiki-developer-tools in core (e.g. just for "dev_phpunit" and "dev_paratest" modes)