Waits for a function to return true (waits for 1sec by default).

```js
I.waitUntil(() => window.requests == 0);
I.waitUntil(() => window.requests == 0, 5);
```

@param {function|string} fn function which is executed in browser context.
@param {number} [sec=1] (optional, `1` by default) time in seconds to wait
@param {string} [timeoutMsg=''] message to show in case of timeout fail.
@param {?number} [interval=null]