test: increase coverage of process.emitWarning

Previously our tests did not check these codepaths as seen at
coverage.nodejs.org

PR-URL: https://github.com/nodejs/node/pull/9556


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>