test: Print stderr when subprocess fails

Summary:
This is a partial backport of core#17517
https://github.com/bitcoin/bitcoin/pull/17517/commits/fa69cef13e5aab8264339eb3d50a9e89d59efd87

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12796