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: #9171
Reviewed-by: Colin Ihrig <cjihrig@gmail.com>
Reviewed-by: Timothy J Fontaine <tjfontaine@gmail.com>

Author: jscissr

test/simple/test-timers-unref.js

@@ -42,7 +42,8 @@ setTimeout(function() {
 interval = setInterval(function() {
   unref_interval = true;
   clearInterval(interval);
-}, SHORT_TIME).unref();
+}, SHORT_TIME);
+interval.unref();
 
 setTimeout(function() {
   unref_timer = true;