Jest Unit Test for rxjs retryWhen method

Question

I've being trying to increase the test's coverage of a project. Looking into the coverage report, I saw that retryWhen method is not being covered and since then I've been struggling.

Here's the function I'm trying to test:

launchWebHook<T>(
    webhookUrl: string,
    webhookRequest: IWebHookShape<T>,
    signedToken: string,
  ): Observable<IWebHookAxiosResponse> {
    const requestOptions = {
      headers: {}, // not empty in src code
    };
    return this.httpService.post(webhookUrl, webhookRequest, requestOptions).pipe(
      retryWhen(errors =>
        errors.pipe(
          delay(900000),
          take(3),
          scan((acc, source, index) => {
            if (index) {
              throw source;
            }
          }),
        ),
      ),
    );
  }

And here is the test I have:

const webhookUrl = 'https://webhook.com';
const webhookRequest = {} as IWebHookShape<string>;
const signedToken = 'signed';
const response = {} as AxiosResponse<any>;

(httpService.post as jest.Mock).mockResolvedValue(new Observable<AxiosResponse<any>>());
const mockPipe = jest.spyOn(Observable.prototype, 'pipe');
const retryWhen = mockPipe.mock.calls[0][1];

webhookService.launchWebHook(webhookUrl, webhookRequest, signedToken);
expect(retryWhen).toHaveBeenCalled();

I tried using a similar implementation of this answer, but couldn't make it work.

I'm also trying to use mockFn.mock.calls based on this answer