test: capture stderr from child processes
If the test fails with errors from the child commands, there is no debug info. Suppliment the stderr data so that we know what to look for.
Refs: https://github.com/nodejs/node/issues/25988
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines