Testing loopback 3 with jest and supertest does not exit gracefully

I am trying to build a test using Jest and Supertest for my loopback 3 rest API server. The test is running, but Jest did not exit gracefully.

This is my test script branch.test.js:

const app = require('../server/server');
const request = require('supertest');

describe('Test the root path', () => {
    afterAll(done => {
        app.close(done);
    });

    test('It should response the GET method', async () => {
        const response = await request(app).get('/api/branches');
        expect(response.statusCode).toBe(200);
    });
})

And package.json for test setting

{
    "scripts": {
        "lint": "eslint .",
        "start": "node .",
        "posttest": "npm run lint",
        "test": "jest --detectOpenHandles"
    },
    "jest": {
        "testEnvironment": "node"
    }
}

Every time I run npm run test, the log always return:

Jest has detected the following 1 open handle potentially keeping Jest from exiting:
● TCPWRAP
11 |
12 | test('It should response the GET method', async () => {
13 | const response = await request(app).get('/api/branches');
14 | expect(response.statusCode).toBe(200);
15 | });
16 | })
at Test.Object..Test.serverAddress (node_modules/supertest/lib/test.js:59:33)
at new Test (node_modules/supertest/lib/test.js:36:12)
at Object.obj.(anonymous function) [as get] (node_modules/supertest/index.js:25:14)
at Object.get (test/unit/branch.test.js:13:45)

the log is pointing at get function. People in forum use jest --forceExit, but it is not the way I want. Are there any solutions for this problem? Thanks for the response