Add new section under testing "Using test waiters"

this is one of the most important topics in testing, and we don't say anything haha

What is settled?
when do you want to be settled?
renderSettled?
some techniques for defining settledness?
- with modifiers (floating-ui, for example)
- async
- observer-gating

how to debug waiters?

```js
// in test-helper.ts
Object.assign(window, { getSettledState, getPendingWaiterState, currentURL, currentRouteName });
// (this should probably be in the blueprint)
```

and then when a test haults, you can use getSettledState, etc