test: update test/parallel/test-eval.js

Changed var -> const and assert.equal -> assert.strictEqual

PR-URL: https://github.com/nodejs/node/pull/8590


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>