Ignore .env.local in test environment (facebook#2250)

Author: gaearon

packages/react-scripts/config/env.js

@@ -27,9 +27,13 @@ if (!NODE_ENV) {
 var dotenvFiles = [
   `${paths.dotenv}.${NODE_ENV}.local`,
   `${paths.dotenv}.${NODE_ENV}`,
-  `${paths.dotenv}.local`,
+  // Don't include `.env.local` for `test` environment
+  // since normally you expect tests to produce the same
+  // results for everyone
+  NODE_ENV !== 'test' && `${paths.dotenv}.local`,
   paths.dotenv,
-];
+].filter(Boolean);
+
 // Load environment variables from .env* files. Suppress warnings using silent
 // if this file is missing. dotenv will never modify any environment variables
 // that have already been set.