test: update test-child-process-recv-handle

change var to const/let where appropriate
use strictEqual instead of equal
call toString on buffers in strictEqual asserts
use common.mustCall on callbacks containing asserts

PR-URL: https://github.com/nodejs/node/pull/8648


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>