如何在rxjs中对retryWhen操作符进行单元测试？

import { of, throwError, timer } from 'rxjs';
import { mergeMap, retryWhen } from 'rxjs/operators';

it('should retry the observable when an error occurs', () => {
  let retryCount = 0;

  const source$ = throwError('Error')
    .pipe(
      retryWhen(errors => errors.pipe(
        mergeMap(error => {
          if (retryCount < 3) {
            retryCount++;
            return timer(1000); // 1秒后重试
          }
          return throwError(error); // 达到最大重试次数后，抛出错误
        })
      ))
    );

  source$.subscribe({
    error: error => {
      expect(error).toBe('Error');
    }
  });
});