Re-write tests for Promise.prototype internal slots

The original implementation of the test for the absense of a
[[PromiseState]] internal slot did not actually assert the documented
semantics. Re-implement the test to rely on the IsPromise abstract
operation (via `Promise.prototype.then`) to accurately ensure that the
object does not have a [[PromiseState]] internal slot.

In relying on the semantics of the `instanceof` operator, the original
test for the [[Prototype]] internal slot was imprecise (the assertion
could be satisfied if additional objects were incorrectly defined on the
prototype chain). Re-write the test to assert the value of the
[[Prototype]] internal slot directly.