feat(cdk/testing): Consider adding a `TestElementPromise` class

#### Feature Description

@yjaaidi proposed this idea in https://github.com/angular/components/issues/20871. We could create a subclass of `Promise` that would allow calling `TestElement` methods directly on the Promise. Example: https://stackblitz.com/edit/test-element-promise

#### Use Case

This would allow people to write cleaner test code, e.g.

```ts
await harness.host().getText();
```

Instead of:

```ts
await (await harness.host()).getText();
// or
await harness.host().then(host => host.getText());
```

The idea would need a design doc and discussion with the team before proceeding.

Start by reviewing the TestElement and harness APIs, then compare the linked proposal with the TestElementPromise example. Produce a design document covering the Promise subclass and direct TestElement method calls, and use team discussion to determine whether the feature should proceed.

Written by the indexing model from the issue text.