test: Timeout#unref() does not return instance

Timeout#unref() call returns undefined, not this. The test already
worked before, because the interval was still unref'd, and the test also
succeeds without clearing the interval.

PR-URL: nodejs/node-v0.x-archive#9171
Reviewed-by: Colin Ihrig <[email protected]>
Reviewed-by: Timothy J Fontaine <[email protected]>

Conflicts:
	test/simple/test-timers-unref.js

Author: jscissr

test/parallel/test-timers-unref.js

@@ -21,7 +21,8 @@ setTimeout(function() {
 interval = setInterval(function() {
   unref_interval = true;
   clearInterval(interval);
-}, SHORT_TIME).unref();
+}, SHORT_TIME);
+interval.unref();
 
 setTimeout(function() {
   unref_timer = true;