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/models/category_user_spec.rb

@@ -68,7 +68,7 @@ describe CategoryUser do
 
   context 'integration' do
     before do
-      SiteSetting.queue_jobs = false
+      run_jobs_synchronously!
       NotificationEmailer.enable
     end