Tests: Use dnssec.works instead of verteiltesysteme.net

[DL6ER]

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Use dnssec.works instead of verteiltesysteme.net for FTL's CI tests as the latter broke (see here for further details). This PR updates the test to use a different DNSSEC validation service: https://dnssec.works/

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/unbound-pi-hole-resolving-with-huge-delay-throwing-4711-hostname-problem/61114/3