test: case sensitivity of env variables

Environment variables should be treated case-insensitive on Windows
platforms and case-sensitive on UNIX platforms.

This commit ensures this behavior persists.

Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/9166
Fixes: https://github.com/nodejs/node/issues/9157