[fix] supertest fails with intermittent "socket hang up" in loops

[ikonst]

When testing a rate limiting feature, I've encountered intermittent low level errors in the form of "socket hang up", etc.

What I found to reliably work around this, is to create a supertest agent for a listening(!) HTTP server rather than an ExpressJS app, i.e.

 const app = express();
 app.get("/", (req, res) => res.send("OK"));
-const superAgent = request(app);
+const server = http.createServer(app).listen(0);
+const superAgent = request(server);

 for (let i = 0; i < 1000; i++) {
   await superAgent.get("/").expect(200);
 }

Maybe it's possible to do the server.listen(0) on the TestAgent level so it doesn't have to be repeated for every request, and thus sometimes fail (e.g. for port allocation reasons).