chore: test cleanup

[jeswr]

Makes use .resolve in jest.

I was actually trying to repro an error that I am seeing in another library where .resolve isn't behaving well with the readable-stream library but seems to work fine here 😕

[rubensworks]

So the change is not really needed then? 😅
Maybe we can leave things as-is then, don't fix what ain't broken?

Also, the conversion of async to sync tests may not be that convenient, in case we'd need to await multiple things in tests in the future.