zlib: tighten up dictionary tests

Uses const where possible, removes inaccurate comments, prefers
strictEqual where possible, ensures functions with assertions are called
and enures the inflater has correct encoding set.

PR-URL: https://github.com/nodejs/node/pull/8512


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>