Nicer way to check how many times a function has been called

[LeaVerou]

This is one of the things hTest needs for parity with other testing frameworks, and can be quite useful for testing events etc.

One way to implement it would be a predefined run function that tests can import, but the count needs to be specific to the test.

Perhaps this can be done if the function maintains a weakmap of Test objects to counts? Or it can also use data to store the count?

But then how do you signal that the test is over? You want to fail when it's called both fewer times, and more times than you want, so using a promise that resolves when it's been called as many times as expect specified doesn't seem to fly.

Looks like we need a timeout for async tests (if we don't already have one), but that by itself is not enough.