test: var variables to const in zlib

Changes in test-zlib-from-string is because var->const
pushed us over the max char limit per line.

PR-URL: https://github.com/nodejs/node/pull/8627


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: targos - Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>