fix: do not depend on external domains in dnslink tests

[lidel]

Dependency on third-party domains means that over time CI will break, like it did in https://github.com/ipfs/kubo/actions/runs/9214249430/job/25364989609?pr=10429#step:9:63

Error: queryCname ENODATA singularity.storage

The three domains I left are stable and under IPFS Foundation / Shipyard control.

Ideally, CI would not depend on internet DNS, and have static fixtures and custom localhost DNS server, but this PR should be good enough to stop breaking CI on other project's PRs like https://github.com/ipfs/kubo/pull/10429

Notes & open questions

Change checklist

• [x] I have performed a self-review of my own code
• [x] I have made corresponding changes to the documentation if necessary (this includes comments as well)
• [x] I have added tests that prove my fix is effective or that my feature works

[SgtPooki]

Fyi current helia-verified-fetch gwc PR (todo link) has some dnsLink stubbing code (IPFS_NS_MAP like functionality)