test: verify hash seed uniqueness

This tests that the hash seed used by V8 for hashing is random.

PR-URL: https://github.com/nodejs/node-private/pull/84


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>