Add test for presence of isSupported property.
Add test for presence of isSupported property.

neilj/DOMPurify,voku/DOMPurify,tdeekens/DOMPurify,Joris-van-der-Wel/DOMPurify,Joris-van-der-Wel/DOMPurify,togusafish/t-ashula-_-DOMPurify,neilj/DOMPurify,tdeekens/DOMPurify,neilj/DOMPurify,ConradIrwin/DOMPurify,kwangkim/DOMPurify,kwangkim/DOMPurify,Joris-van-der-Wel/DOMPurify,tdeekens/DOMPurify,ConradIrwin/DOMPurify,voku/DOMPurify,togusafish/t-ashula-_-DOMPurify