MDL-48373 behat: new container selector for list items.
MDL-48373 behat: find table columns using contains() logic.
MDL-48373 behat: generators for questions and categories
MDL-48373 behat, questions: convert tests to use the new steps

The new steps make it more efficient to create questions.

While making the changes, I took the opportunity to alter the tests to
follow Behat best practices, and only test one thing per scenario.