DEV: Try fix flaky topic view serializer spec (#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.

spec/serializers/topic_view_serializer_spec.rb

@@ -57,7 +57,7 @@ describe TopicViewSerializer do
         expect(json[:image_url]).to end_with(image_upload.url)
       end
 
-      it 'should have thumbnails' do
+      it 'should have thumbnail jobs enqueued' do
         SiteSetting.create_thumbnails = true
 
         Discourse.redis.del(topic.thumbnail_job_redis_key(Topic.thumbnail_sizes))
@@ -72,6 +72,16 @@ describe TopicViewSerializer do
         expect do
           json = serialize_topic(topic, user)
         end.to change { Jobs::GenerateTopicThumbnails.jobs.size }.by(0)
+      end
+
+      it 'should have thumbnails after jobs run' do
+        Jobs.run_immediately!
+        SiteSetting.create_thumbnails = true
+
+        Discourse.redis.del(topic.thumbnail_job_redis_key(Topic.thumbnail_sizes))
+        json = serialize_topic(topic, user)
+        topic.generate_thumbnails!
+        json = serialize_topic(topic, user)
 
         # Original + Optimized
         expect(json[:thumbnails].length).to eq(2)