When calling setImmediate with extra arguments the this keyword in the
callback would refer to the global object, but when not calling
setImmediate with extra arguments this would refer to the returned
handle object.
This commit fixes that inconsistency so its always set handle object.
The handle object was chosen for performance reasons.
The test would fail if the child process writes anything to the stdout.
This doesn't happen on unix, since `cat` is spawned. However, on Windows
`cmd` is started, which *does* write stuff to it's stdout. This
meanlingless assert is now removed.