Make the Take Quiz e2e tests more flexible

The sample quiz that's used for the Take Quiz e2e tests has been moved into a separate JSON file. The e2e prep script (prepare.js) uses it, and takeQuiz.spec.js now also loads it in and uses it. Added some helper methods to submit the current question either correctly or incorrectly.

There should now be far fewer things to update if I add questions to the sample quiz. When testing something specific, such as whether the alternative answers are correctly accepted for fill-in questions, I still use hardcoded values for clarity, but for general things like quiz navigation, I'm now using the helpers.
Take Quiz: Show supplemental info after question submission