test: assert cluster.disconnect is async

See joyent/node#8043, test passed on v0.11 already, but this makes the
test stronger.

Reviewed-by: Trevor Norris <trev.norris@gmail.com>

test/simple/test-cluster-disconnect-with-no-workers.js

@@ -31,3 +31,6 @@ process.on('exit', function() {
 cluster.disconnect(function() {
   disconnected = true;
 });
+
+// Assert that callback is not sometimes synchronous
+assert(!disconnected);