fix: dns test

[richarddavison]

Description of changes

Test introduced by #825 had assertions in callbacks of non-asynchronous functions. This will (by design) crash the process if not handled correctly.

Throwing an error in a callback will result in process exit. This PR fixes the assertions by wrapping the lookup in a promise. Another option would be to use the done callback and an inner try catch in the callback.

Checklist

• Created unit tests in tests/unit and/or in Rust for my feature if needed
• Ran make fix to format JS and apply Clippy auto fixes
• Made sure my code didn't add any additional warnings: make check
• Added relevant type info in types/ directory
• Updated documentation if needed (API.md/README.md/Other)

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