docs(aio): add NgModule docs (#20306)

PR Close #20306

aio/content/examples/providers/src/app/app.component.spec.ts

@@ -0,0 +1,32 @@
+import { TestBed, async } from '@angular/core/testing';
+import { AppComponent } from './app.component';
+
+describe('AppComponent', () => {
+  beforeEach(() => {
+    TestBed.configureTestingModule({
+      declarations: [
+        AppComponent
+      ],
+    });
+    TestBed.compileComponents();
+  });
+
+  it('should create the app', async(() => {
+    const fixture = TestBed.createComponent(AppComponent);
+    const app = fixture.debugElement.componentInstance;
+    expect(app).toBeTruthy();
+  }));
+
+  it(`should have as title 'app works!'`, async(() => {
+    const fixture = TestBed.createComponent(AppComponent);
+    const app = fixture.debugElement.componentInstance;
+    expect(app.title).toEqual('app works!');
+  }));
+
+  it('should render title in a h1 tag', async(() => {
+    const fixture = TestBed.createComponent(AppComponent);
+    fixture.detectChanges();
+    const compiled = fixture.debugElement.nativeElement;
+    expect(compiled.querySelector('h1').textContent).toContain('app works!');
+  }));
+});