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/user_spec.rb

@@ -1088,7 +1088,7 @@ describe User do
 
       context "with a reply" do
         before do
-          SiteSetting.queue_jobs = false
+          run_jobs_synchronously!
           PostCreator.new(Fabricate(:user),
                             raw: 'whatever this is a raw post',
                             topic_id: topic.id,