Bring test bootstrap file in line with drupal 10.2

I've temporarily updated the carrot.json in this PR too so this can be tested with 3 different versions. Should remove that after.

The problem is in drupal 10.2 they've changed where it expects things to be found. In particular all tests fail with Symfony\Component\DependencyInjection\Exception\InvalidArgumentException: The service file "core/core.services.yml" is not valid, which is a misleading error that actually means "file doesn't exist", because it's expecting a relative path from DRUPAL_ROOT/web.