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

test/parallel/test-process-env.js

@@ -1,6 +1,6 @@
 'use strict';
 
-require('../common');
+const common = require('../common');
 const assert = require('assert');
 
 // changes in environment should be visible to child processes
@@ -67,3 +67,17 @@ assert.equal(3, date.getUTCHours());
 assert.equal(5, date.getHours());
 */
 /* eslint-enable max-len */
+
+// Environment variables should be case-insensitive on Windows, and
+// case-sensitive on other platforms.
+process.env.TEST = 'test';
+assert.strictEqual(process.env.TEST, 'test');
+// Check both mixed case and lower case, to avoid any regressions that might
+// simply convert input to lower case.
+if (common.isWindows) {
+  assert.strictEqual(process.env.test, 'test');
+  assert.strictEqual(process.env.teST, 'test');
+} else {
+  assert.strictEqual(process.env.test, undefined);
+  assert.strictEqual(process.env.teST, undefined);
+}