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.

plugins/discourse-local-dates/spec/models/post_spec.rb

@@ -3,7 +3,7 @@ require 'rails_helper'
 describe Post do
 
   before do
-    SiteSetting.queue_jobs = false
+    run_jobs_synchronously!
   end
 
   describe '#local_dates' do