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.

lib/generators/plugin/templates/controller_spec.rb.erb

@@ -2,7 +2,7 @@ require 'rails_helper'
 
 describe <%= name %>::ActionsController do
   before do
-    SiteSetting.queue_jobs = false
+    run_jobs_synchronously!
   end
 
   it 'can list' do