How can I enable IDE integration (autocomplete) for WordPress test suite

In order of preference:

1. composer require wp-phpunit/wp-phpunit --dev. Depending on the IDE you use you might need to mark the package in vendor/ as a source directory.
2. Each IDE allows you to add definitions from external libraries. In PHPStorm it’s Settings → Languages & Frameworks → PHP → Include Path.
3. Just copy the files over to some directory in your project and exclude them with .gitignore (if you use Git for version control).