Improve promise iteration logic

[ehmicky]

This PR improves promise[Symbol.asyncIterator] logic, fixing a few bugs including:

• Currently, each stdout line waits for one stderr line, and vice-versa
• The underlying iterator.return() and iterator.throw() are not called, which means try { ... } finally { await resultPromise } was not called when calling for await (line of nanoSpawn(...)) { ...; break; }

The PR also adds more tests.