DEV: Try fix flaky topic view serializer spec (PR #13601)

This is just a hunch, but this is quite a complex test. I think that there is some timing issue where the jobs enqueued for generating the thumbnails via the serializer thumbnails method and they aren’t generated in time before we do the json[:thumbnails] check. Split the tests up into two, with one checking the right jobs are enqueued and another with Jobs.run_immediately! that checks that json[:thumbnails] is correct.