Fix Sidekiq warnings about JSON serialization (#17381)

* Fix Sidekiq warnings about JSON serialization

This occurs on every symbol argument we pass, and every symbol key in hashes,
because Sidekiq expects strings instead.

See https://github.com/mperham/sidekiq/pull/5071

We do not need to change how workers parse their arguments because this has
not changed and we were already converting to symbols adequately or using
`with_indifferent_access`.

* Set Sidekiq to raise on unsafe arguments in test mode

In order to more easily catch issues that would produce warnings in production
code.

app/workers/activitypub/distribution_worker.rb

@@ -27,6 +27,6 @@ class ActivityPub::DistributionWorker < ActivityPub::RawDistributionWorker
   end
 
   def options
-    { synchronize_followers: @status.private_visibility? }
+    { 'synchronize_followers' => @status.private_visibility? }
   end
 end