message: docs: clarify where the `async` utility function is coming from
  inside the `beforeEach` for out testing guide.
  Fixes
  PR Close
diff: @@ -618,6 +618,16 @@ It also generates an initial test file for the component, `banner-external.compo
  region="v1"
  header="app/banner/banner-external.component.spec.ts (initial)"></code-example>
  +<div class="alert is-helpful">
  +
  +Because `compileComponents` is asynchronous, it uses
  +the [`async`](api/core/testing/async) utility
  +function imported from `@angular/core/testing`.
  +
  +Please refer to the [async](#async) section for more details.
  +
  +</div>
  +
  #### Reduce the setup
  Only the last three lines of this file actually test the component
  