如何测试在angular中是否调用了subscriber？

import { ComponentFixture, TestBed } from '@angular/core/testing';
import { Component } from '@angular/core';
import { Observable } from 'rxjs';

@Component({
  selector: 'app-test',
  template: '',
})
class TestComponent {
  observable$: Observable<any>;

  methodToTest() {
    this.observable$.subscribe();
  }
}

describe('TestComponent', () => {
  let component: TestComponent;
  let fixture: ComponentFixture<TestComponent>;

  beforeEach(async () => {
    await TestBed.configureTestingModule({
      declarations: [TestComponent],
    }).compileComponents();
  });

  beforeEach(() => {
    fixture = TestBed.createComponent(TestComponent);
    component = fixture.componentInstance;
    fixture.detectChanges();
  });

  it('should call subscriber', () => {
    spyOn(component.observable$, 'subscribe');
    component.methodToTest();
    expect(component.observable$.subscribe).toHaveBeenCalled();
  });
});