test: use common.isWindows consistently

In the tests, we use "process.platform === 'win32'" in some places.
This patch replaces them with the "common.isWindows" for consistency.

PR-URL: https://github.com/nodejs/io.js/pull/2269


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>