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/categories_controller_spec.rb

@@ -78,7 +78,7 @@ describe CategoriesController do
 
     describe "logged in" do
       before do
-        SiteSetting.queue_jobs = false
+        run_jobs_synchronously!
         sign_in(admin)
       end
 
@@ -226,7 +226,7 @@ describe CategoriesController do
 
   context '#update' do
     before do
-      SiteSetting.queue_jobs = false
+      run_jobs_synchronously!
     end
 
     it "requires the user to be logged in" do