test: fix stderr reference

PR-URL: #15938
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

Author: osukaa

test/parallel/test-child-process-silent.js

@@ -41,7 +41,7 @@ if (process.argv[2] === 'pipe') {
     stdoutData = true;
   });
   let stderrData = false;
-  parent.stdout.on('data', function() {
+  parent.stderr.on('data', function() {
     stderrData = true;
   });