Nodejs API test throwing Socket Hang up error with SuperTest - node.js

Question

I'm trying to add unit tests to my nodejs application using mocha.
I've created a dummy test endpoint '/test' which returns a string as shown below :
Test endpoint :
app.get(routeHelper.getURLPath('/test'), (req, res, next) => {
res.send('Helloo from test route !!');
next();
});
sample.test.js :
process.env.NODE_ENV = 'test';
let supertest = require("supertest");
let should = require("should");
let server = supertest.agent("http://localhost:9000");
describe('Routes', () => {
it('sample test', (done) => {
server
.get("/test")
.expect(200)
.end(function(err,res){
// console.log('***************Ressssssss******* ', res.body);
console.log('***************errrrrrrrrrrrrrr******* ', err);
res.status.should.equal(200);
res.body.error.should.equal(false);
done();
});
});
});
When I run the test, it is throwing the error shown below :
{ Error: socket hang up
at createHangUpError (_http_client.js:322:15)
at Socket.socketOnEnd (_http_client.js:425:23)
at Socket.emit (events.js:187:15)
at endReadableNT (_stream_readable.js:1094:12)
at process._tickCallback (internal/process/next_tick.js:63:19) code: 'ECONNRESET', response: undefined }
Accessing the API directly from the browser is giving the response properly. But, it's throwing this error when I run the test.
Any help to fix this will very helpful.