Set NODE_ENV to to 'test' if not already set

Fixes avajs#1470

lib/fork.js

@@ -10,12 +10,10 @@ if (fs.realpathSync(__filename) !== __filename) {
 	console.warn('WARNING: `npm link ava` and the `--preserve-symlink` flag are incompatible. We have detected that AVA is linked via `npm link`, and that you are using either an early version of Node 6, or the `--preserve-symlink` flag. This breaks AVA. You should upgrade to Node 6.2.0+, avoid the `--preserve-symlink` flag, or avoid using `npm link ava`.');
 }
 
-let env = process.env;
+const env = Object.assign({NODE_ENV: 'test'}, process.env);
 
 // Ensure NODE_PATH paths are absolute
 if (env.NODE_PATH) {
-	env = Object.assign({}, env);
-
 	env.NODE_PATH = env.NODE_PATH
 		.split(path.delimiter)
 		.map(x => path.resolve(x))