Extending supertest on typescript

I'm trying to create a extension on supertest.

Using what I found in question Extending SuperTest. I have this working example on javascript:

const request = require('supertest');
const Test = request.Test;

Test.prototype.authenticate = function(user) {
  const {token, xsrfToken} = user.tokens;

  return this
   .set('Authorization', `Bearer ${token}`)
   .set('X-XSRF-TOKEN', xsrfToken);
}

And inside a test block I can use:

request(app)
  .post('/user/settings')
  .authenticate(user)
  .send(...)

This works fine. The problem now is to use the extension in a *.test.ts file.

As suggested in Extend Express Request object using Typescript, I try to create a file to use the typescript feature Declaration Merging.

// file location src/types/supertest

declare namespace supertest {
  export interface Test {
    authenticate(user: any): this; // I didn't put a type on user to simplify here.
  }
}

and also changed my tsconfig.json

{
  "compilerOptions": {

    ...

    "typeRoots": ["./src/types"],

    ...

  }
}

But when I run npx tsc

$ npx tsc
src/api/user.test.ts:51:8 - error TS2551: Property 'authenticate' does not exist on type 'Test'.

51       .authenticate(user);
          ~~~~~~~

Is there a way to fix this, use this aproach in typescript?