test(cdk/scrolling): Change test spying on tick

tick is not called by schedulers anymore (they use an internal _tick)

Author: atscott

src/cdk/scrolling/virtual-scroll-viewport.spec.ts

@@ -820,19 +820,17 @@ describe('CdkVirtualScrollViewport', () => {
         appRef = TestBed.inject(ApplicationRef);
       });
 
-      it('should not run change detection if there are no viewChange listeners', fakeAsync(() => {
+      it('should not emit viewChange if there are no listeners', fakeAsync(() => {
+        const viewChangeSpy = spyOn(testComponent.virtualForOf.viewChange, 'next');
         finishInit(fixture);
         testComponent.items = Array(10).fill(0);
         fixture.changeDetectorRef.markForCheck();
         fixture.detectChanges();
         flush();
 
-        spyOn(appRef, 'tick');
-
         viewport.scrollToIndex(5);
         triggerScroll(viewport);
-
-        expect(appRef.tick).not.toHaveBeenCalled();
+        expect(viewChangeSpy).not.toHaveBeenCalled();
       }));
     });
   });