Add support for promises in `expect`

Right now, if you have a promise in expect (for, say, an async function) it will be run but errors will not be detected.

[Fundebug]

@LegNeato It seems the problem is not solved.

I‘m using supertest@4.0.2.

I find that the AssertionError will not be detected when I use async/await in expect.

 .expect(async res => {
     const orderId = res.body.orderId;
     const order = await Order.findById(orderId).lean(); // { price: 10 }

     // the AssertionError will not be detected
     order.should.deepEqual({
         price: 50
     });
 });

[LegNeato]

I don’t know, it could have regressed since this was landed years ago. I only use Rust these days so sadly can’t help.