Promise: Add tests to disallow faulty optimization

Add tests that assert behavior when a Promise is resolved with another
Promise whose `then` method has been overridden. Because all objects
with a `then` method are treated equivalently, the presence of a
[[PromiseState]] internal slot should have no effect on program
behavior.

These tests guard against a faulty optimization originally implemented
in V8:

https://bugs.chromium.org/p/v8/issues/detail?id=3641