test: move test-specific function out of common

common.checkSpawnSyncRet is only used in one test. Move it out of
common.js and into that test (test-child-process-spawnsync-input.js).

PR-URL: https://github.com/nodejs/node/pull/3871


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