test: add IPv6 brackets but no port to test-dns

Add a test case to test-dns to check that supply an IPv6 host with
brackets but no explicit port to `dns.setServers()` yields expected
results. This is the final bit of test coverage missing for
lib/internal/dns/utils.js.

PR-URL: https://github.com/nodejs/node/pull/27006


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>