test: refactored test-crypto-random.js

Replaced `var` by `const` for constant variables, enforced
strict equality check, and replaced custom callback execution
check by `common.mustCall`.

PR-URL: https://github.com/nodejs/node/pull/8632


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>