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/requests/embed_controller_spec.rb

@@ -74,7 +74,7 @@ describe EmbedController do
     let(:headers) { { 'REFERER' => embed_url } }
 
     before do
-      SiteSetting.queue_jobs = false
+      run_jobs_synchronously!
     end
 
     it "raises an error with no referer" do