use npm.common instead of spawn In the process, followed failing code path to command that wasn't respecting the loglevel and updated to RESPECT

Tests that call spawn() and ensure that no stdout/stderr events occur don't seem to work. In particular, the last npm prune test did not catch the stderr event.
This change needs to be removed – this output is part of the unbuild command's explicit contract, and will always be printed out on success. This is practically holy write from @isaacs, and you will see this pattern in most of npm's commands.