test: refactor test-tls-0-dns-altname

* var -> const, let
* assert.equal() -> assert.strictEqual()

PR-URL: https://github.com/nodejs/node/pull/9948


Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: James M Snell <jasnell@gmail.com>