test: show stdout and stderr in test-cli-syntax when it fails

To help debugging the flake with the log from the CI.

PR-URL: #24720
Refs: #24403
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-cli-syntax.js

@@ -34,6 +34,12 @@ const notFoundRE = /^Error: Cannot find module/m;
 
     const cmd = [node, ..._args].join(' ');
     exec(cmd, common.mustCall((err, stdout, stderr) => {
+      if (err) {
+        console.log('-- stdout --');
+        console.log(stdout);
+        console.log('-- stderr --');
+        console.log(stderr);
+      }
       assert.ifError(err);
       assert.strictEqual(stdout, '');
       assert.strictEqual(stderr, '');