MDL-48373 behat: find table columns better.

It was not possible to select the question bank 'Created by'
or 'Modified by' columns without this change.
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.