bug(TestbedHarnessEnvironment): Impossible to use if component under test uses intervals

Use StackBlitz to reproduce your issue: **ADDED** (thanks @annieyw!), [example here](https://stackblitz.com/edit/material-testbedharness-interval?file=src/app/button-harness-example.spec.ts)

Steps to reproduce:
1. Follow [the instructions for using Harnesses](https://material.angular.io/guide/using-component-harnesses)
2. In the component under test, use `setInterval`, or the rxjs `interval(n)` or `timer(0, n)` observables
3. Your test will stall indefinitely at e.g. `await loader.getAllHarnesses(MatButtonHarness);` (from the docs linked in step 1)

#### Expected Behavior

`getAllHarnesses` completes once the component has finished initializing and had a chance to create instances of child components.

#### Actual Behavior

The call to `getAllHarnesses` never resolves, and the test times out.

#### Environment

- Angular: 10.1.2
- CDK/Material: 10.2.1
- Browser(s): N/A
- Operating System (e.g. Windows, macOS, Ubuntu): N/A (Win10)

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As I understand it, this is because under the hood, `TestbedHarnessEnvironment` is using `ComponentFixture#whenStable`. From what I can tell, [the official stance from Angular / Zone](https://github.com/angular/angular/issues/10127) is that you should use `fakeAsync` plus `tick`, and call `discardPeriodicTasks` to ignore the (expected) pending intervals, instead of using `waitForAsync` / `whenStable`.

This [very old StackOverflow question](https://stackoverflow.com/questions/38697656/testing-angular2-components-that-use-setinterval-or-settimeout) suggests that one should never have `setInterval` in a component or service, instead using an injected service to provide periodic actions, so it can be mocked out during testing, but I've never seen anything "official" from Angular to say that intervals are unsupported.

If it's not permissible to use intervals in a component, this should be documented somewhere, with suggestions of alternative design patterns to use that are compatible with Material's test utilities. If it is permissible, `TestbedHarnessEnvironment` should be updated to include another way of determining when the owning `ComponentFixture` is "ready", or I guess kick this upstream to the Zone folks so they can make `whenStable` work with intervals.

Comienza con la reproducción en StackBlitz y el punto de entrada TestbedHarnessEnvironment, centrándote en su uso de ComponentFixture#whenStable. Compara el comportamiento con las guías de Angular y Zone enlazadas sobre fakeAsync, tick y discardPeriodicTasks. Se considera terminado cuando el caso del intervalo tenga un comportamiento compatible acordado, ya sea una ruta funcional del harness o documentación que explique la limitación y el patrón de pruebas compatible.

Escrito por el modelo de indexación a partir del texto del issue.