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/components/post_destroyer_spec.rb

@@ -617,7 +617,7 @@ describe PostDestroyer do
 
   context '@mentions' do
     it 'removes notifications when deleted' do
-      SiteSetting.queue_jobs = false
+      run_jobs_synchronously!
       user = Fabricate(:evil_trout)
       post = create_post(raw: 'Hello @eviltrout')
       expect {