fs.promises.symlink() does not return the right Promise

[addaleax]

fs.promises.symlink() returns a Promise for the FSReqPromise itself, not the actual async operation that is in progress.

I think all that's needed is a req_wrap->SetReturnValue() call in Symlink() in node_file.cc, but then again it seems like we should maybe refactor the args.GetReturnValue().Set() call in AsyncDestCall() to already do that?

(Noticed this while looking into local test failures on Windows -- developing there is a pain, so I'm not fixing this/opening a PR, so this is up for grabs.)

[jasnell]

I'll take a look :-)

[joyeecheung]

Should be fixed in #18811