TestCafe: How to test logout if user logged in from another machine

I am glad I asked.

In order to test if a login in another browser triggers a logout in the current browser, there is no need to run a different browser. You can send the according login command from your test code.

node.js builtin standard http library is sufficient for that task. The official documentation has a specific section on http requests: https://nodejs.org/en/knowledge/HTTP/clients/how-to-create-a-HTTP-request/

I personally prefer the fetch API as available in the browser. node-fetch provides this API in node.

so your test code could look a little like this:

import 'node-fetch';
import { URLSearchParams } from 'url';

// we assume we get page state and interaction from this seperate module
import { loginAction, getIsLogged } from './page-actions';

fixture `login logut`
    .page `http://your.app/`;

test('User is logged out if logged in somewhere else', async t => {
    // perform the login actions to login as "username"
    await loginAction(t, 'yourUsername', 'yourPassword');
    
    await simulateLoginFromSomewhereElse('yourUsername', 'yourPassword');

    await t.expect(getIsLoggedIn(t)).eql(false);
});

async function simulateLoginFromSomewhereElse(username, password) {
    // build the (form) data to be sent to the server
    const params = new URLSearchParams();
    params.append('username', 'yourUsername');
    params.append('password', 'yourPassword');

    await fetch(`http://your.app/login`, { method: 'POST', body: params });
}