feat(@angular/cdk/testing): Component Harness Feedback

**This is an open discussion aiming to regroup Component Harness feedback and improvement ideas.**

Most of the items below are focused on simplifying the API. The current APIs are a bit too complex; this can have a negative impact on `TestHarness` adoption.

# For test authors

## 1. Easier access to harness

`HarnessLoader` is a nice abstraction but it can make harness instantiation cumbersome.

### Actual approach
```ts
let fixture: ComponentFixture;
let loader: HarnessLoader;
let rootLoader: HarnessLoader;

beforeEach(() => {
fixture = TestBed.createComponent(MyDialogButton);
loader = TestbedHarnessEnvironment.loader(fixture);
rootLoader = TestbedHarnessEnvironment.documentRootLoader(fixture);
});

it('loads harnesses', async () => {
const dialogButtonHarness = await TestbedHarnessEnvironment.harnessForFixture(fixture, MyDialogButtonHarness);

const buttonHarness = await loader.getHarness(MyButtonHarness);
});
```

It would be nice to have faster harness access like:

### Suggestion 1.A
```ts
let fixture: ComponentFixture;

beforeEach(() => {
fixture = TestBed.createComponent(MyDialogButton);
});

it('loads harnesses', async () => {
const dialogButtonHarness = await TestbedHarnessEnvironment.getHarness(MyDialogButtonHarness, {fixture});

const buttonHarness = await TestbedHarnessEnvironment.getHarness(MyButtonHarness, {fixture});
});
```

or even global functions like `getHarness` and `getProtractorHarness` functions could make the tests even more readable:

### Suggestion 1.B
```ts
let fixture: ComponentFixture;

beforeEach(() => {
fixture = TestBed.createComponent(MyDialogButton);
});

it('loads harnesses', async () => {
const dialogButtonHarness = await getHarness(MyDialogButtonHarness, {fixture});
const buttonHarness = await getHarness(MyButtonHarness, {fixture});
});
```

# For harness authors

## 2. `LocatorFactory` abstraction

The `LocatorFactory` approach _(e.g. `locatorFor()` method returns a function that takes no parameters)_ can be confusing and cumbersome.

### Actual approach
```ts
class MyPopupHarness extends ComponentHarness {
static hostSelector = 'my-popup';

protected getTriggerElement = this.locatorFor('button');

async toggle() {
const trigger = await this.getTriggerElement();
return trigger.click();
}

}
```

### Suggestion 2.A

Simple accessor methods like `get()` or `getOptional()` seem easier to use and more intuitive.
```ts
class MyPopupHarness extends ComponentHarness {
static hostSelector = 'my-popup';

async toggle() {
const trigger = await this.get('button');
return trigger.click();
}
}
```

We can let developers factorize the way they want:
```ts
getTriggerElement() {
return this.get('button');
}
```

## 3. `async / await` vs chaining

I am personally not a big fan of chaining (a.k.a. builder pattern) but in cases like this one where we end up with lots of `await`s, this can simplify the interface:

### Actual approach
```ts
async isDisabled() {
const el = await this.getMessageElement();
const text = await el.text();
return text === 'Disabled';
}
```

### Suggestion 3.A
```ts
async isDisabled() {
return (await this.getMessageElement().text()) === 'Disabled';
}
```

## 4. Trigger any event

`TestElement` should have a `triggerEvent` function that allows harness authors to trigger any event.

### Suggestion 4.A
```ts
el.triggerEvent('dragenter', {})
```

# Common

## 5. Provide synchronous functions

Some environments can query the DOM synchronously (e.g. `TestbedHarnessEnvironment`) or through some under the hood chaining (e.g. Cypress) (Cf. https://docs.cypress.io/guides/core-concepts/introduction-to-cypress.html#Chains-of-Commands).
Harness authors might want to focus on these environments. In that case, they will want to use synchronous functions and keep tests and harnesses easier to read & write.

### Current approach

```ts
class ItemListHarness extends ComponentHarness {
static hostSelector = 'app-item-list';

getItems = this.locatorForAll('li');

async getItemNames() {
const items = await this.getItems();
const itemNames = await Promise.all(items.map(item => item.text()));
return itemNames;
}
}

it('loads harnesses', async () => {
const itemListHarness = await loader.getHarness(ItemListHarness);
expect(await itemListHarness.getItemNames()).toEqual(['🍔', '🍟']);
});
```

### Suggestion 5.A

Providing synchronous alternatives to accessors.

```ts
class ItemListHarness extends ComponentHarness {
static hostSelector = 'app-item-list';

getItemNamesSync() {
return this.getAllSync('li').map(item => item.textSync());
}
}

it('loads harnesses', () => {
const itemListHarness = loader.getHarnessSync(ItemListHarness);
expect(itemListHarness.getItemNamesSync()).toEqual(['🍔', '🍟']);
});
```

## 6. `TestbedHarnessEnvironment ` vs. `TestBedHarnessEnvironment `

`TestbedHarnessEnvironment` could be renamed to `TestBedHarnessEnvironment` to stay consistent with `TestBed` 😉

## 7. Merge `TestBed` and `TestbedHarnessEnvironment`

In some future, wouldn't it be nice to merge `TestbedHarnessEnvironment` with `TestBed` which means moving test harness to the angular repo?

## 8. Cypress support

An external library could provide a `CypressHarnessEnvironment` but as presented in the 5th item, Cypress is based on an abstract chain of commands. `TestElement` doesn't seem to be the right abstraction for this use case especially for getters like `text()`, `getProperty()` etc...

This is the last item on the list but probably the most important one. One of the key features of harnesses is the test environment abstraction and harness reuse through environments (TestBed, Protractor etc...) but if I am using TestBed and Cypress and if I can't reuse my harnesses with Cypress then it somewhat defeats the purpose of harnesses.

没有指定具体的文件、测试或入口点。首先查看 Component Harness APIs 和八个独立的提案；在开始实现之前，需要将此次讨论缩小为一项已确定且具有明确定义完成标准的变更。

由索引模型根据 Issue 内容生成。