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/address_endpoint_factory.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :address_endpoint do
+    server
+    sequence(:address) { |n| "test#{n}@example.com" }
+  end
+end