feat(material/testing): Extend Angular harness testing functionality

[wildcardalice]

This pull request adds the following functionalities to the harness test kit

getHarnessAtIndex

This new function works acts like getHarness, but returns an instance of the harness corresponding to the matching element at the given index. This throws an error if the index is out of bounds.

Example usage:

// Retrieves the 4th option on screen
const option = await loader.getHarnessAtIndex(MatOptionHarness, 3);

countHarnesses

This new function counts the number of matching component instances and returns the result.

Example usage:

// Expect there to be five options on the screen
expect(await loader.countHarnesses(MatOptionHarness)).toBe(5);

Filter by Label

New functionality makes it possible to fetch harnesses for certain form components using their floating label text. Previously, the user would have to either label the component with an id or class, or locate it in a form field using MatFormFieldHarness.getControl().

This affects the following harnesses:

• MatInputHarness
• MatSelectHarness
• MatNativeSelectHarness
• MatDatepickerInputHarness
• MatDateRangeInputHarness

Example usage:

// template
<mat-form-field>
  <mat-label>Favorite food</mat-label>
  <input matInput value="Pizza"/>
</mat-form-field>

// test.ts
const input = await loader.getHarness(MatInputHarness.with({label: 'Favorite food'}));

[mmalerba]

should we switch this to target: minor considering it missed the feature freeze for v20?