Allow `waitUntil` to accept a callback that returns a promise

[boris-petrov]

Right now, waitUntil expects that the callback returns a truthy/falsy value. It would be nice to allow the callback to return a promise which resolves to a truthy/falsy value. So something like to work (and return after 1 second not immediately like now because the returned promise is a truthy value):

await waitUntil(() => new Promise((resolve) => setTimeout(() => resolve(true), 1000)));

[rishiraj88]

Thanks for this, @boris-petrov .