Fixed flaky batch sending tests (#21604)

ref https://linear.app/ghost/issue/ENG-1749

Batch sending tests were failing with MySQL fairly regularly. It appears
to be a race condition where the listener for the batch sending job
having completed is returning too early, causing the subsequent
Bookshelf data model refresh to happen too soon.

This is a fundamental flaw in the JobManager awaitCompletion handler
(and how the batch sending system interacts with it) as there's no way
to identify one batch from another - they all use the same name, and we
don't pass along any metadata.

ghost/core/test/integration/services/email-service/batch-sending.test.js

@@ -109,6 +109,7 @@ describe('Batch sending tests', function () {
             value: false
         }], {context: {internal: true}});
         mockManager.restore();
+        await jobManager.allSettled();
     });
 
     before(async function () {