Fix tests for customized built-in elements in custom-elements/htmlcon…

…structor/newtarget.html (web-platform-tests#4082)

According to the spec, https://html.spec.whatwg.org/multipage/dom.html#htmlconstructor,
the fallback prototype of customized built-in element is the interface prototype object
from NewTarget's realm, instead of the interface object.

custom-elements/htmlconstructor/newtarget.html

@@ -101,9 +101,9 @@
     function TestElement() {
       const o = Reflect.construct(w.HTMLParagraphElement, [], new.target);
 
-      assert_equals(Object.getPrototypeOf(o), window.HTMLParagraphElement,
+      assert_equals(Object.getPrototypeOf(o), window.HTMLParagraphElement.prototype,
         "Must use the HTMLParagraphElement from the realm of NewTarget");
-      assert_not_equals(Object.getPrototypeOf(o), w.HTMLParagraphElement,
+      assert_not_equals(Object.getPrototypeOf(o), w.HTMLParagraphElement.prototype,
         "Must not use the HTMLParagraphElement from the realm of the active function object (w.HTMLParagraphElement)");
 
       return o;