test: improve tests in pummel/test-exec

* add asset.strictEqual to stdout and stderr. Fixes no-unused-vars.
* replace double quotes with single

PR-URL: https://github.com/nodejs/node/pull/10757


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>