Quiz/question Behat improvements: better quiz building, and question selector

These are two Behat improvements I need for MDL-40990, but I am submitting them for integration separately for clarity.

1. In the 'quiz "Quiz 1" contains the following questions:' step, I am switching to having explicity table headings in the table. This is because we are about to add more possibly columns, and having unlabelled columns is a pain, and almost unreadable.

2. I have added new 'question' selector, so you can say things like 'And I click on the "Check" "button" in the "What is a cat?" "question".

Not sure it was good to combine these two into one bug, but it happened. Sorry.