test: add tests for message unqueueing

This adds a comprehensive set of tests for the message unqueueing service.

Additionally, it improves how message databases are used for testing environments.

spec/factories/server_factory.rb

@@ -53,5 +53,10 @@ FactoryBot.define do
     trait :suspended do
       suspended_at { Time.current }
     end
+
+    trait :exceeded_send_limit do
+      send_limit_approaching_at { 5.minutes.ago }
+      send_limit_exceeded_at { 1.minute.ago }
+    end
   end
 end