test: show child stderr output in largepages test

The test starts child processes. A recent change is suspected of causing
flaky crashes on one of the alpine buildbots but we can't know for sure
because the test hides the child's stderr.

Refs: https://github.com/nodejs/node/pull/31547#issuecomment-581076515

PR-URL: https://github.com/nodejs/node/pull/31612
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

test/parallel/test-startup-large-pages.js

@@ -8,7 +8,8 @@ const { spawnSync } = require('child_process');
 
 {
   const child = spawnSync(process.execPath,
-                          [ '--use-largepages=on', '-p', '42' ]);
+                          [ '--use-largepages=on', '-p', '42' ],
+                          { stdio: ['inherit', 'pipe', 'inherit'] });
   const stdout = child.stdout.toString().match(/\S+/g);
   assert.strictEqual(child.status, 0);
   assert.strictEqual(child.signal, null);