Bug 821604 - Don't start periodic sync until we have a calendar that needs a real sync operation r=millermedeiros

I would remove the comment comment and do simply:

```js
var accounts = app.store('Account');
sinon.stub(accounts, 'syncableAccounts')
  .returns(Promise.resolve([ {} ]));
```

and later:

```js
sinon.assert.calledOnce(accounts.syncableAccounts);
accounts.syncableAccounts.restore()
```