test: refactor the code in test-dns-ipv4

* remove the manual control for functions execution
* use common.mustCall to control the functions execution automatically
* use let and const instead of var
* use assert.strictEqual instead assert.equal

PR-URL: https://github.com/nodejs/node/pull/10200


Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>