Spectron with mocha and chai does not work

I am trying to write a tests with Spectron for our Electron App, but I am running into problems with the setup. I use the classical setup with chai. I have one file which contains setup code:

const path = require("path");
const { Application } = require("spectron");

module.exports = {
  initialiseSpectron: () => {
    let electronPath = path.join(__dirname, "../../node_modules", ".bin", "electron");

    if (process.platform == "win32") {
      electronPath += ".cmd";
    }

    return new Application({
      path: electronPath,
      args: [path.join(__dirname, "../index.ts"), path.join(__dirname, "../../package.json")],
      env: {
        ELECTRON_ENABLE_LOGGING: true,
        ELECTRON_ENABLE_STACK_DUMPING: true,
        NODE_ENV: "development"
      },
      startTimeout: 10000,
      chromeDriverLogPath: "../chromedriverlog.txt"
    });
  },
  sleep: time => new Promise(resolve => setTimeout(resolve, time))
};

And then the test itself:

const chaiAsPromised = require("chai-as-promised");
const chai = require("chai");
chai.should();
chai.use(chaiAsPromised);

const testHelper = require("./initialise");
const app = testHelper.initialiseSpectron();

// Setup Promises and start Application
before(() => app.start());

// Tear down App after Tests are finished
after(() => {
  if (app && app.isRunning()) {
    return app.stop();
  }
});

describe("Login", () => {
  it("opens a window", function() {
    return app.client
      .waitUntilWindowLoaded()
      .getWindowCount()
      .should.eventually.equal(1);
  });

  it("tests the title", () =>
    app.client
      .waitUntilWindowLoaded()
      .getTitle()
      .should.eventually.equal("VIPFY"));
});

My problem is that I always get this error:

 1) "before all" hook in "{root}"

  0 passing (2s)
  1 failing

  1) "before all" hook in "{root}":
     Error: Timeout of 2000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.

So it looks like the App does not start. But that is not true. The App Window opens, but it seems like the test does not recognize that. I have already tried changing the path using all kinds of syntax. But nothing worked. What am I missing?