Jest --detectleaks shows memory leak while using SuperTest with Express #634

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?