test: increase coverage in lib/internal/dns/promises.js

Add a test for the only uncovered code in lib/internal/dns/promises.js.

PR-URL: https://github.com/nodejs/node/pull/27330


Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>