fix: don’t let timeouts prevent child tests to run

If a test is supposed to be implicitly ended by its children a timer will fail it and prevent its children to run.

lib/test.js

@@ -120,6 +120,7 @@ Test.prototype.timeoutAfter = function(ms) {
         self.fail('test timed out after ' + ms + 'ms');
         self.end();
     }, ms);
+    timeout.unref();
     this.once('end', function() {
         clearTimeout(timeout);
     });