test: refactor `assert` in internet test-dns.js

change assert.equal() to assert.strictEqual()
and use assert.strictEqual() for type validation

PR-URL: https://github.com/nodejs/node/pull/8980


Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>