test: cover DNS entrypoint discovery

[dkropachev]

Summary

• Fixes test if dns lb actually works #22.
• Add discovery unit coverage for a DNS hostname seed entrypoint and DNS hostname records returned from /localnodes.
• Add HTTP integration coverage that discovers through a resolver-backed localhost entrypoint using live /localnodes data.

Tests

• npm test -- --run test/discovery.test.ts -t "resolves DNS entrypoint"
• npx eslint test/discovery.test.ts test/integration-test/alternator-client.test.ts
• INTEGRATION_TESTS=false npm run test:integration -- --run test/integration-test/alternator-client.test.ts -t "DNS entrypoint"
• git diff --check