Add test for AbortSignal.abort()

See whatwg/dom#960.

Author: jasnell

dom/abort/event.any.js

@@ -64,4 +64,9 @@ test(t => {
   controller.abort();
 }, "the abort event should have the right properties");
 
+test(t => {
+  const signal = AbortSignal.abort();
+  assert_true(signal.aborted);
+}, "the AbortSignal.abort() static returns an already aborted signal");
+
 done();