`assertPromiseResolved()`

**Is your feature request related to a problem? Please describe.**

During tests I often want to verify that a promise has been resolved right at this moment.

**Describe the solution you'd like**

This is a function that I've been using a lot for this:
```js
/**
* Asserts whether a promise is currently resolved or not. The check is made asynchronously.
* The call waits for the next event loops and gives the promise a chance to resolve in the current event loop.
* The reason for this is that there is no way to synchronously check the resolved state of promises in JavaScript.
* @param {Promise} promise
* @param {boolean} expected
*/
export async function assertPromiseResolved(promise, expected) {
let resolved = false;
(async () => {
await promise;
resolved = true;
})();
await waitForMicrotasks();
const msg = expected ? "Expected the promise to be resolved" : "Expected the promise to not be resolved";
assert(resolved == expected, msg);
}
```

`waitForMicrotasks` is a very basic:
```js
export function waitForMicrotasks() {
return new Promise(r => setTimeout(r, 0));
}
```
As far as I'm aware this is required, there's no way to figure out the state of promises synchronously.

Is this something that would be suitable for deno_std? I've got a few tests for this as well.

**Describe alternatives you've considered**

Keep using this from my own repository ¯\\\_(ツ)_/¯