This test consists in checking whether each <optgroup> in a <select> tag has a "label" attribute.
Dans chaque liste de choix (balise select), chaque regroupement d'items de liste (balise optgroup) possède-t-il un attribut label ?
At least one <optgroup> has no "label" attribute (Test1 returns false for at least one element).
All the <optgroup> tags have "label" attribute (Test1 returns true for all the elements of Set1).