Error response: WebSockets request was expected

I have an almost identical test case to https://github.com/visionmedia/supertest/issues/566 , except in my test I'm iterating over the `await request().get().expect()` 1000 times, to test the performance of the endpoint.

80% of the time, the tests pass, but occasionally I hit the following issue.

> Error: expected '{"mydata":0}' response body, got 'WebSockets request was expected\r\n'

(400 Bad Request)

Has anyone seen this before?

I've tried adding Promise awaits for the handlers in the beforeEach/afterEach, to no avail.

---

Node: v18.3.0
Express: 4.17.1
Supertest: 6.2.4

TestCase.ts
```javascript
import express, { Application } from 'express'
import { Server } from 'http'
import { describe, it } from 'mocha'
import request from 'supertest'
import MyRouter from '../../src/routes/MyRouter'

describe('wip PerformanceTesting - 1000x should be less than X milliseconds', () => {
const app: Application = express().use(MyRouter)
let server: Server

beforeEach(async () => {
server = await new Promise((resolve) => {
const s = app.listen(0, () => resolve(s))
})
})

afterEach(async () => {
await new Promise((resolve) => server.close(() => resolve(null)))
})

it('my performance test', async () => {
for (let i = 0; i < 1000; i++) await request(app).get('/api/sales/data').expect('{"mydata":0}').expect(200)
}).timeout(1000)
})

```

Start with the repeated request case in TestCase.ts, using MyRouter and the /api/sales/data endpoint, then inspect the beforeEach and afterEach server lifecycle. Reproduce the intermittent 400 response with the listed Node, Express, and Supertest versions; done means explaining why some of the 1000 requests receive the WebSockets response and establishing reliable expected responses.

Written by the indexing model from the issue text.