test: pass through stderr in benchmark tests

This helps a lot with debugging failing benchmark tests,
which would otherwise just print an assertion for the
exit code (something like `+1 -0`, which yields almost no
information about a failure).

PR-URL: #21860
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>

Author: addaleax

test/common/benchmark.js

@@ -20,7 +20,10 @@ function runBenchmark(name, args, env) {
 
   const mergedEnv = Object.assign({}, process.env, env);
 
-  const child = fork(runjs, argv, { env: mergedEnv, stdio: 'pipe' });
+  const child = fork(runjs, argv, {
+    env: mergedEnv,
+    stdio: ['inherit', 'pipe', 'inherit', 'ipc']
+  });
   child.stdout.setEncoding('utf8');
 
   let stdout = '';