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/discourse_single_sign_on_spec.rb

@@ -8,7 +8,7 @@ describe DiscourseSingleSignOn do
     SiteSetting.sso_url = @sso_url
     SiteSetting.enable_sso = true
     SiteSetting.sso_secret = @sso_secret
-    SiteSetting.queue_jobs = false
+    run_jobs_synchronously!
   end
 
   def make_sso