Test several functions callbacks with mocha/chai

I have a global object which has the ability to assign functions for events, like:

obj.on('event', () => {});

After exact public API is called these events are fired as well.

Now I need to write async tests with mocha.js/chai.js and run it in the node.js environment.

I stuck in a situation when two events subscriptions should be tested at once.

All code is written in TypeScript and later transpiled into JavaScript.

The code in a global object:

public someEvent(val1: string, val2: Object) {
 // some stuff here...
 this.emit('event_one', val1);
 this.emit('event_two', val1, val2);
}

The code in a test file (my latest realization):

// prerequisites are here...
describe('test some public API', () => {
 it('should receive a string and an object', (done) => {
  // counting number of succesfull calls
  let steps = 0;

  // function which will finish the test
  const finish = () => {
   if ((++steps) === 2) {
    done();
   }
  };

  // mock values
  const testObj = {
   val: 'test value'
  };

  const testStr = 'test string';

  // add test handlers
  obj.on('event_one', (key) => {
   assert.equal(typeof key, 'string');
   finish();
  });

  obj.on('event_two', (key, event) => {
   assert.equal(typeof key, 'string');
   expect(event).to.be.an.instanceOf(Object);
   finish();
  });

  // fire the event
  obj.someEvent(testStr, testObj);
 });
});

So, my question is - is there any built-in functionality to make this test to look more elegant?

Another question is how to provide some meaningful error information instead of default error string?

Error: Timeout of 2000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.