How to solve ECONNREFUSED error in Supertest and jest with NodeJS?

I am writing test cases of a backend project in NodeJS with MongoDB by using Jest, Supertest and Mongoose for endpoints. Currently I am having the error UnhandledPromiseRejectionWarning: Error: connect ECONNREFUSED 127.0.0.1:60524 and tests are getting failed. The code below is written in asynchronous method which gives actual result I expected. If I write the same logic with then().catch(), there's no error and it shows everything works even it should be the failed test case.

#gen.test.js

const request = require("supertest");
let server;
const { Genres } = require("../../models/genres");

describe("/api/genres", () => {
  beforeEach(() => {
    server = require("../../index");
  });

  afterEach(() => {
    server.close();
    Genres.deleteMany({}).then((res) =>
      winston.info(`Deleted Documents ${res.deletedCount}.`)
    );
  });

  describe("GET /", () => {
    it("should return all the genres.", async () => {
      await Genres.collection.insertMany([
        { name: "genre1" },
        { name: "genre2" },
        { name: "genre3" },
      ]);

      const res = await request(server).get("/api/genres");
      expect(res.status).toBe(200);
      expect(res.body.length).toBe(3);
      expect(res.body.some((g) => g.name === "genre1")).toBeTruthy();

      
    });

  });

    //This one works with then().cath() . But If I make this to something that I don't expect, It still works.

   describe("GET /id", () => {
    it("should return a single genre", () => {
      const genre = new Genres({ name: "genre1" });
      genre
        .save()
        .then((res) => winston.info("new genre is saved for /id search"))
        .catch((err) => winston.error(err.message));

      request(server)
        .get("/api/genres/" + genre._id)
        .then((res) => {
          expect(res.status).toBe(200);
          expect(res.body).toHaveProperty("name", genre.name);
        });
    });
});

I got the error from the Jest is

 ● /api/genres › GET / › should return all the genres.

    expect(received).toBe(expected) // Object.is equality

    Expected: 200
    Received: 500

      26 |
      27 |       const res = await request(server).get("/api/genres");
    > 28 |       expect(res.status).toBe(200);
         |                          ^
      29 |       expect(res.body.length).toBe(3);
      30 |       expect(res.body.some((g) => g.name === "genre1")).toBeTruthy();
      31 |

      at Object.<anonymous> (tests/integration/genres.test.js:28:26)

and UnhandledPromiseRejectionWarning Errors are repeated.

(node:31169) UnhandledPromiseRejectionWarning: Error: connect ECONNREFUSED 127.0.0.1:60524
    at TCPConnectWrap.afterConnect [as oncomplete] (net.js:1141:16)
(node:31169) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:31169) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
(node:31169) UnhandledPromiseRejectionWarning: Error: connect ECONNREFUSED 127.0.0.1:60525
    at TCPConnectWrap.afterConnect [as oncomplete] (net.js:1141:16)
(node:31169) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 2)
(node:31169) UnhandledPromiseRejectionWarning: Error: connect ECONNREFUSED 127.0.0.1:60528
    at TCPConnectWrap.afterConnect [as oncomplete] (net.js:1141:16)
(node:31169) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 3)
(node:31169) UnhandledPromiseRejectionWarning: Error: connect ECONNREFUSED 127.0.0.1:60529
    at TCPConnectWrap.afterConnect [as oncomplete] (net.js:1141:16)
(node:31169) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 4)
A worker process has failed to exit gracefully and has been force exited. This is likely caused by tests leaking due to improper teardown. Try running with --detectOpenHandles to find leaks.

I have been doing research on this so long to fix this error. How can I solve this issue?