dns: make `var` to `const` if possible
Refs: https://github.com/nodejs/node/pull/13732
Checklist
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make -j4 test
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commit message follows commit guidelines
Affected core subsystem(s)
dns
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Refs: https://github.com/nodejs/node/pull/13732
make -j4 test
dns