Jest testing with Node - Timeout - Async callback was not invoked within the 5000ms timeout specified by jest.setTimeout

You can expect asynchronous results using resolves, as shown in the Jest documentation.

In your case:

it('should get a list of posts', function() {
    const result = posts.fetchPosts();
    expect(result).resolves.toEqual(expect.any(Object));
})

…although I have a suspicion your list of posts is actually an array, so you probably want this:

it('should get a list of posts', function() {
    const result = posts.fetchPosts();
    expect(result).resolves.toEqual(expect.any(Array));
})

Another tip: You don't need to wrap the body of your fetchPost in an additional promise, you can simply return the promise you get from Posts.find and add a then to it, like this:

module.exports = {
    fetchPosts() {
        return Posts.find({}).then(posts => {
            if (posts) {
                return posts;
            } 
            throw new Error('no posts'); // this will cause a promise rejection
        })
    }
}

It's also highly possible that you're not getting a response back from the DB at all from your test suite. Test suite's can call different environmental variables / configs that lead to different calls. This error can also be seen if no response is returned, as in - if someone blocks your IP from connecting, on and on.