test: refactor test-zlib.js

* minor layout changes for clarity
* assert.equal() and assert.ok() swapped out for assert.strictEqual()
* var -> const for modules included via require()

PR-URL: https://github.com/nodejs/node/pull/9544


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>