Reference how to test, and remove unneeded step (GoogleCloudPlatform#1754)

Author: engelke

appengine/storage/standard/README.md

@@ -38,3 +38,7 @@ Then set environment variables before starting your application:
  export GCLOUD_STORAGE_BUCKET=<your-bucket-name>
  npm install
  npm start
+
+Run the tests, if desired:
+
+ npm test

appengine/storage/standard/system-test/app.test.js

@@ -31,10 +31,6 @@ before(async () => {
  process.env.GCLOUD_PROJECT,
  `Must set GCLOUD_PROJECT environment variable!`
  );
- assert(
- process.env.GOOGLE_APPLICATION_CREDENTIALS,
- `Must set GOOGLE_APPLICATION_CREDENTIALS environment variable!`
- );
  await bucket.create(bucket).then(() => {
  return bucket.acl.add({
  entity: 'allUsers',