Tesntig a method that uses observable on the constructor

I need to cover this method with Jasmine in my code

private subject = new Subject<any>();

getMessage(): Observable<any> {
    return this.subject.asObservable();
  }

Here is my constructor

fiscalDocument: FiscalDocumentData;
subscription: Subscription;

constructor(private _myFiscalDocumentsService: MyFiscalDocumentsService) {  
    this.subscription = this._myFiscalDocumentsService.getMessage().subscribe(message => {
      if (message) {
        this.fiscalDocument = message;

      } else {
        this.fiscalDocument = null;
      } 
    });

  }

I've already tried to do that in my test, I have the TestBed.confiugreTestingModule, but I won't post here because I believe that's my problem isn't there

let dummyOb: Observable<any>;

  beforeEach(() => {
    service = TestBed.get(MyFiscalDocumentsService);
    fixture = TestBed.createComponent(MyFiscalDocumentsDetailsComponent);
    component = fixture.componentInstance;
    fixture.detectChanges();
});

it('should getMessage', async () => {
    let generalSpy = spyOn(service, 'getMessage' ).and.returnValue(await Promise.resolve(dummyOb));
    expect(generalSpy).toHaveBeenCalled();
 });

The error message: Expected spy getMessage to have been called.