ES2022 compatible version of: https://material.angular.io/components/stepper/examples#stepper-harness

my.component.spec.ts

import { HarnessLoader, parallel } from '@angular/cdk/testing';
import { TestbedHarnessEnvironment } from '@angular/cdk/testing/testbed';
import { ComponentFixture, TestBed, waitForAsync } from '@angular/core/testing';
import { MatStepperModule } from '@angular/material/stepper';
import {
  MatStepHarness,
  MatStepperHarness,
  MatStepperNextHarness,
  MatStepperPreviousHarness,
} from '@angular/material/stepper/testing';
import { NoopAnimationsModule } from '@angular/platform-browser/animations';
import { TestComponent } from './test.component';

describe('TestComponent', () => {
  let fixture: ComponentFixture<TestComponent>;
  let loader: HarnessLoader;
  let stepper: MatStepperHarness;
  let steps: MatStepHarness[];

  beforeEach(waitForAsync(() => {
    TestBed.configureTestingModule({
      imports: [NoopAnimationsModule, MatStepperModule],
      declarations: [TestComponent],
    })
      .compileComponents()
      .then(() => {
        fixture = TestBed.createComponent(TestComponent);
        loader = TestbedHarnessEnvironment.loader(fixture);
        loader.getHarness(MatStepperHarness).then(async (harness) => {
          stepper = harness;
          steps = await stepper.getSteps();
        });
      });
  }));

  it('should get the steps of a stepper', () => {
    expect(steps.length).toEqual(3);
  });

  it('should be able to get the template-based label of a step', async () => {
    expect(
      await parallel(() => {
        return steps.map((step) => step.getLabel());
      })
    ).toEqual(['One', 'Two', 'Three']);
  });

  it('should go forward when pressing the next button', async () => {
    const [, secondStep] = steps;
    const nextButton = await secondStep.getHarness(MatStepperNextHarness);

    await secondStep.select();

    expect(
      await parallel(() => steps.map((step) => step.isSelected()))
    ).toEqual([false, true, false]);

    await nextButton.click();

    expect(
      await parallel(() => steps.map((step) => step.isSelected()))
    ).toEqual([false, false, true]);
  });

  it('should go back when pressing the back button', async () => {
    const [, secondStep] = steps;
    const backButton = await secondStep.getHarness(MatStepperPreviousHarness);

    await secondStep.select();

    expect(
      await parallel(() => steps.map((step) => step.isSelected()))
    ).toEqual([false, true, false]);

    await backButton.click();

    expect(
      await parallel(() => steps.map((step) => step.isSelected()))
    ).toEqual([true, false, false]);
  });
});