test: increase coverage for exec() functions

This commit increases code coverage related to the stdout
and stderr outputs of the child_process exec() functions.
Previously, stdout was completely covered, but stderr was
not.

PR-URL: https://github.com/nodejs/node/pull/10919


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>