test: add coverage for string array dgram send()

This commit adds code coverage for dgram's Socket#send() where
the data is an array of strings.

PR-URL: https://github.com/nodejs/node/pull/11247


Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>