Error in Angular asynchronous test with Jest

I want to test a service with jest, but I'm having trouble testing an asynchronous function that gives a get in my api.

The error I get is as follows:

FAIL  tests/documentos.service.spec.ts (11.205 s)
  DocumentosService
    × encountered a declaration exception (518 ms)

  ● DocumentosService › encountered a declaration exception

    Cannot call Promise.then from within a sync test.

Function in service:

async listarDocumentos():Promise<Documento[]>{
    let documentos: Documento[];
    await this.http.get<Documento[]>(`${urlDocumentos}`, {observe:'body'}).toPromise()
    .then(res =>{
      if(res.length <= 0) throw 'Não existem documentos cadastrados'
      documentos = res
    })
    .catch((error:HttpErrorResponse) =>{
      this.mensagem.error(`Erro ao listar documentos: ${error.message}`,`Erro: ${error.status || 'Listar Documentos'}`)
    })
    return documentos
  }

Test file:

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

import { DocumentosService } from '../src/app/service/documentos/documentos.service';

import { DocumentosDoSistemaComponent } from '../src/app/documentos-do-sistema/documentos-do-sistema.component';

import { Documento } from '../src/app/service/documentos/documentos.model';

let componenteFake: DocumentosDoSistemaComponent;
let fixture: ComponentFixture<DocumentosDoSistemaComponent>;
let documentosService: DocumentosService;

describe('DocumentosService', () => {
  beforeEach(() => TestBed.configureTestingModule({
    declarations:[DocumentosDoSistemaComponent],
    providers:[DocumentosService]
  }));

  fixture = TestBed.createComponent(DocumentosDoSistemaComponent);
  componenteFake = fixture.componentInstance;
  documentosService = TestBed.get(DocumentosService);

  test('Teste do service de documentos', async () =>{
    let documentos:Documento[];
    documentos = await documentosService.listarDocumentos();
    expect(documentos.length).toBeGreaterThan(0);
  })

});