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

还没有人认领这个 Issue。

#### 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.

首先审查 TestElement 和 harness API，然后将链接的提案与 TestElementPromise 示例进行比较。编写一份设计文档，涵盖 Promise 子类和直接调用 TestElement 方法，并通过团队讨论确定是否应继续推进该功能。

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