Support `resolves` or `rejects` (assertions for promises)

[Pet3ris]

Jest seems to have good support for promises with assertions: https://jestjs.io/docs/en/asynchronous#promises, which may be useful to provide.

[glennsl]

There's already testPromise. Do you think that's not sufficient?

[Pet3ris]

@glennsl it doesn't seem as nice / aligned syntactically but functionally sufficient for sure!

[glennsl]

I'm not really a fan of offering multiple ways of doing something, but since it's a pattern in jest, and as long as it's feasible, I'm not opposed to it. And at least API-wise it seems like it should line up well with the bs-jest approach of requiring a single assertion to be returned.