I don’t know why is that task mentioned in the guide.
The test database schema is maintained transparently for you. The normal workflow is:
bundle exec rake
Done. That performs all the necessary setup and runs the suite. (If the development database has pending migrations, you need to run db:migrate in dev mode as usual.)
The setup is also automated for the fine-grained test tasks.
It is very rare that you need to run any db:* task by hand to run the suite.