How to use the rxjs-marbles/mocha.fakeSchedulers function in rxjs-marbles

it(
    "should support a timer",
    fakeSchedulers(() => {
      let received: number | undefined;
      timer(100).subscribe(value => (received = value));
      clock.tick(50);
      expect(received).to.be.undefined;
      clock.tick(50);
      expect(received).to.equal(0);
    })
  );

  it(
    "should support delay",
    fakeSchedulers(() => {
      let received: number | undefined;
      of(1)
        .pipe(delay(100))
        .subscribe(value => (received = value));
      clock.tick(50);
      expect(received).to.be.undefined;
      clock.tick(50);
      expect(received).to.equal(1);
    })
  );

  afterEach(() => {
    clock.restore();
  });
});