Jest --detectleaks shows memory leak while using SuperTest with Express

Hey everyone,

I have been experiencing memory leaks in any test files that require supertest when using Jest with the `--detectLeaks` option on an express server.

### Versions
node: **12.16.1**
node: **6.14.2**

### Modules
jest: **25.1.0**
supertest: **4.0.2**
weak-napi: **1.0.3** _(jest complains without this module available when using `detectLeaks`)_
express: **4.17.1**

To isolate the issue from any custom development code in my particular application, I have created this repository to highlight the problem:

https://github.com/fakekamrankhan/express-supertest-jest-detect-memory-leak

If you run `npm test` you'll see the memory leak warning. Removing **supertest** gets rid of the memory leak flag. The test.js file in question:
```
const express = require('express');
const request = require('supertest');
describe('leak', () => {

let port;
let server;

beforeAll(() => {
port = 3000;
server = new express().listen(port);
});

afterAll(() => {
server.close();
})
test('leak test', async () => {
let res = await request(server).get('/');
expect(res.status).toBe(404);
});
});
```

The error:
```
EXPERIMENTAL FEATURE!
Your test suite is leaking memory. Please ensure all references are cleaned.

There is a number of things that can leak memory:
- Async operations that have not finished (e.g. fs.readFile).
- Timers not properly mocked (e.g. setInterval, setTimeout).
- Keeping references to the global scope.

at onResult (node_modules/@jest/core/build/TestScheduler.js:188:18)
```

I have been wracking by brain for the last four days trying to pinpoint the issue. Does anyone know if this could be anything in the test or if this is a bug in **superest** or **jest**?