test: add test for zlib.create*Raw()

Currently, there are no tests that exercise `zlib.createInflateRaw()` or
`zlib.createDeflateRaw()`.

This adds minimal tests that invoke the functions and confirm that they
return `zlib.InflateRaw`/`zlib.DeflateRaw` objects.

PR-URL: https://github.com/nodejs/node/pull/8306
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-zlib-create-raw.js

@@ -0,0 +1,15 @@
+'use strict';
+
+require('../common');
+const assert = require('assert');
+const zlib = require('zlib');
+
+{
+  const inflateRaw = zlib.createInflateRaw();
+  assert(inflateRaw instanceof zlib.InflateRaw);
+}
+
+{
+  const deflateRaw = zlib.createDeflateRaw();
+  assert(deflateRaw instanceof zlib.DeflateRaw);
+}