为什么所有角度组件测试示例都显示了运行组件测试的低效率方式？

Question

文档中的所有测试在执行每个测试之前都要重新配置测试平台。当然，以下内容必须更有效，并且意味着测试运行速度会大大提高。

由于某些原因，由于区域错误，beforeAll设置测试台和beforeEach创建夹具均不起作用，但是嵌套的描述模式起作用。

describe('The shared form component:', () => {
    let fixture: ComponentFixture<FormComponent>;
    let component: FormComponent;
    let httpClient: HttpClient;
    let httpTestingController: HttpTestingController;

    // setup testbed before all tests run to improve tests speed
    beforeEach(async(() => {
        TestBed.configureTestingModule({
            imports: [
                SharedTestModule,
                FormsModule,
                ReactiveFormsModule,
            ],
            declarations: [
                DateFieldsInputComponent,
                PasswordValidatorDirective,
                FormComponent,
            ],
            providers: [
                { provide: ApiBaseService, useValue: {} }
            ]
        }).compileComponents();

        httpClient = TestBed.get(HttpClient);
        httpTestingController = TestBed.get(HttpTestingController);
    }));

    describe('A basic form with text input fields', () => {

        beforeEach(async(() => {
            fixture = TestBed.createComponent(FormComponent);
            component = fixture.componentInstance;
        }));

        afterEach(() => {
            // After every test, assert that there are no more pending requests.
            httpTestingController.verify();
        });

        it('should create the component', () => {
            expect(component).toBeTruthy();
        });
    });
});