test: cleanup test-c-ares.js

Replace equal with strictEqual, use const instead of var
Replace throw error with assert.ifError

PR-URL: https://github.com/nodejs/node/pull/8577


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>