tests: +sample item

In complement to #133 (closed), I forgot one more element: a sample/demonstration item before the actual test.

Could a special item be added first, just after the code input, to demonstrate the procedure? It should look exactly like the other items, maybe with a specific mention en:"Sample"/fr:"Exemple" but without recording or scoring the response.

Ideally, we should set an ID for a sample item in surveys.csv (e.g., item_id=1 -> I will define it with sound and pictures), so a special column in surveys.csv for this sample item.