add test example for harness

apps/example-app/src/app/examples/20-test-harness.spec.ts

@@ -0,0 +1,31 @@
+import { TestbedHarnessEnvironment } from '@angular/cdk/testing/testbed';
+import { MatButtonHarness } from '@angular/material/button/testing';
+import { MatSnackBarHarness } from '@angular/material/snack-bar/testing';
+import { render, screen } from '@testing-library/angular';
+import user from '@testing-library/user-event';
+
+import { SnackBarComponent } from './20-test-harness';
+
+test('can be used with TestHarness', async () => {
+  const view = await render(SnackBarComponent);
+  const loader = TestbedHarnessEnvironment.documentRootLoader(view.fixture);
+
+  const buttonHarness = await loader.getHarness(MatButtonHarness);
+  const button = await buttonHarness.host();
+  button.click();
+
+  const snackbarHarness = await loader.getHarness(MatSnackBarHarness);
+  expect(await snackbarHarness.getMessage()).toMatch(/Pizza Party!!!/i);
+});
+
+test('can be used in combination with TestHarness', async () => {
+  const view = await render(SnackBarComponent);
+  const loader = TestbedHarnessEnvironment.documentRootLoader(view.fixture);
+
+  user.click(screen.getByRole('button'));
+
+  const snackbarHarness = await loader.getHarness(MatSnackBarHarness);
+  expect(await snackbarHarness.getMessage()).toMatch(/Pizza Party!!!/i);
+
+  expect(screen.getByText(/Pizza Party!!!/i)).toBeInTheDocument();
+});

apps/example-app/src/app/examples/20-test-harness.ts

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+import { Component } from '@angular/core';
+import { MatButtonModule } from '@angular/material/button';
+import { MatSnackBar, MatSnackBarModule } from '@angular/material/snack-bar';
+
+@Component({
+  selector: 'app-harness',
+  standalone: true,
+  imports: [MatButtonModule, MatSnackBarModule],
+  template: `
+    <button mat-stroked-button (click)="openSnackBar()" aria-label="Show an example snack-bar">Pizza party</button>
+  `,
+})
+export class SnackBarComponent {
+  constructor(private snackBar: MatSnackBar) {}
+
+  openSnackBar() {
+    return this.snackBar.open('Pizza Party!!!');
+  }
+}