如何为打开模态的Angular5 HttpInterceptor编写单元测试？

@Injectable()
export class ErrorInterceptor implements HttpInterceptor {
  constructor(private _ngbModal: NgbModal) {}

  intercept(request: HttpRequest<any>,next: HttpHandler): Observable<HttpEvent<any>> {
    return next.handle(request)
    .catch((err) => {
      let componentInstance = this._ngbModal.open(ErrormodalComponent,{size: 'sm',backdrop: 'static' }).componentInstance

      switch(err.status) {
        case 401:
          componentInstance["message"] = "User session lost,please logout and log back in."
          break;
        case 404:
          componentInstance["message"] = "API Route Not found"
          break;
        default:
          componentInstance["message"] = "API Currently Unavailable. Please try again."
          break;


      }
      return Observable.throw(err);
    })
  }
}

我想编写一个测试,检查模式是否已根据状态使用适当的消息打开.

谢谢