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>