test: pipe some error output if npm fails

This test now prints out some child error output if the npm child proc
fails, allowing us to debug easier.

PR-URL: https://github.com/nodejs/node/pull/12490
Refs: https://github.com/nodejs/node/pull/12480


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>