Callback → Coroutine Adapter

Some APIs use error-first callbacks: callback(err, result=None).
Implement promisify(fn) that wraps such a function into one you can
await, using an asyncio.Future — the same idea as Node's util.promisify.

await promisify(fn)(...args) should:

• Resolve with result if the callback is invoked as callback(None, result).
• Raise the error if the callback is invoked as callback(err).

solve drives your implementation through a series of test cases and
returns whether each resolved or raised.