lib,test: update let to const where applicable

As per the `prefer-const` eslint rule, few instances of `let` have been
identified to be better with `const`. This patch updates all those
instances.

Refer: https://github.com/nodejs/node/issues/3118
PR-URL: https://github.com/nodejs/node/pull/3152


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>