fix(testing): add discardPeriodicTasks to be used with fakeAsync (#8629)

Closes #8616

modules/@angular/core/testing/fake_async.ts

@@ -80,6 +80,15 @@ export function tick(millis: number = 0): void {
   _getFakeAsyncZoneSpec().tick(millis);
 }
 
+/**
+ * Discard all remaining periodic tasks.
+ */
+export function discardPeriodicTasks(): void {
+  let zoneSpec = _getFakeAsyncZoneSpec();
+  let pendingTimers = zoneSpec.pendingPeriodicTimers;
+  zoneSpec.pendingPeriodicTimers.length = 0;
+}
+
 /**
  * Flush any pending microtasks.
  */