Add resume() for async tests that trap exceptions

[juandopazo]

Hi!

Mocha's async support is awesome, but it as a small deficiency for testing code that catches exceptions, most notably promise-based code or code using task.js. For example:

it('should show the correct assertion failure', function (done) {
  somePromise.then(function (value) {
    // since then() catches exceptions here
    // when the test passes everything is ok
    // but when the assertion fails
    // the test doesn't report the assertion failure
    expect(value).to.be(5);
    done();
  })
})

This pull request introduces a helper function hidden as a property of the done callback that catches its own exceptions and concludes the test the same way as done. This would let us use the following pattern:

it('should show the correct assertion failure', function (then) {
  somePromise.then(function (value) {
    then.resume(function () {
      expect(value).to.be(5);
    });
  })
})

I would be very much OK with other names for resume like next, or a second callback if you think it would be a better choice.

[tomasdev]

Why not leave the logic on the test itself?

it('should show the correct assertion failure', function (done) {
  somePromise.then(function (value) {
    try {
      expect(value).to.be(5);
      done();
    } catch (e) {
      done(e);
    }
  });
});

[juandopazo]

To avoid repetition. I wish there was a less verbose way of doing it, but there doesn't seem to be one.

[juandopazo]

I just realized I can write this on top of done() with a global function and it's nice enough:

function resume(done, fn) {
  try {
    fn();
    done();
  } catch (err) {
    done(err);
  }
}

it('...', function (test) {
  promise.then(function (value) {
    resume(test, function () {
      expect(value).to.be(5);
    })
  })
})