Add new run_jobs_synchronously! helper for tests

Previously if you wanted to have jobs execute in test mode, you'd have
to do `SiteSetting.queue_jobs = false`, because the opposite of queue
is to execute.

I found this very confusing, so I created a test helper called
`run_jobs_synchronously!` which is much more clear about what it does.

spec/components/post_creator_spec.rb

@@ -902,7 +902,7 @@ describe PostCreator do
     end
 
     it 'can post to a group correctly' do
-      SiteSetting.queue_jobs = false
+      run_jobs_synchronously!
 
       expect(post.topic.archetype).to eq(Archetype.private_message)
       expect(post.topic.topic_allowed_users.count).to eq(1)