test: fix flaky test-dns and test-dns-lookup

Ubuntu 16.04 is going to be unsupported after April 2021. Switching
Node.js CI to Ubuntu 18.04 for the internet tests resulted in failures
in test-dns and test-dns-lookup because it returns server failure/try
again on .invalid domain. Add a hostname for testing that will return
record not found.

PR-URL: https://github.com/nodejs/node/pull/38282


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>