test(docs-infra): destroy all `ScrollService` instances after each test (#33937)

`ScrollService` subscribes to global `window` events and mutates global
state in the listener (e.g. read/write values from/to `sessionStorage`).
Therefore, we need to always call its `ngOnDestroy()` method to
unsubscribe from these events after each test.

In f69c6e204, a new testcase was introduced that was not destroyed. As a
result, random failures started to randomly happen in other, unrelated
tests ([example CI failure][1]).

This commit fixes this by ensuring all `ScrollService` instances are
destroyed after each tests (provided that they are created with the
`createScrollService()` helper).

[1]: https://circleci.com/gh/angular/angular/533298

PR Close #33937

aio/src/app/shared/scroll.service.spec.ts

@@ -7,6 +7,13 @@ import { fakeAsync, tick } from '@angular/core/testing';
 import { ScrollService, topMargin } from './scroll.service';
 
 describe('ScrollService', () => {
+  const scrollServiceInstances: ScrollService[] = [];
+  const createScrollService = (...args: ConstructorParameters<typeof ScrollService>) => {
+    const instance = new ScrollService(...args);
+    scrollServiceInstances.push(instance);
+    return instance;
+  };
+
   const topOfPageElem = {} as Element;
   let injector: ReflectiveInjector;
   let document: MockDocument;
@@ -36,7 +43,11 @@ describe('ScrollService', () => {
 
   beforeEach(() => {
     injector = ReflectiveInjector.resolveAndCreate([
-        ScrollService,
+        {
+          provide: ScrollService,
+          useFactory: createScrollService,
+          deps: [DOCUMENT, PlatformLocation, ViewportScroller, Location],
+        },
         { provide: Location, useClass: SpyLocation },
         { provide: DOCUMENT, useClass: MockDocument },
         { provide: PlatformLocation, useClass: MockPlatformLocation },
@@ -51,7 +62,7 @@ describe('ScrollService', () => {
     spyOn(window, 'scrollBy');
   });
 
-  afterEach(() => scrollService.ngOnDestroy());
+  afterEach(() => scrollServiceInstances.forEach(instance => instance.ngOnDestroy()));
 
   it('should debounce `updateScrollPositonInHistory()`', fakeAsync(() => {
     const updateScrollPositionInHistorySpy = spyOn(scrollService, 'updateScrollPositionInHistory');
@@ -100,7 +111,7 @@ describe('ScrollService', () => {
 
     expect(() => {
       const platformLoc = platformLocation as PlatformLocation;
-      const service = new ScrollService(document, platformLoc, viewportScrollerStub, location);
+      const service = createScrollService(document, platformLoc, viewportScrollerStub, location);
 
       service.updateScrollLocationHref();
       expect(service.getStoredScrollLocationHref()).toBeNull();