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/smtp_endpoint_factory.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :smtp_endpoint do
+    server
+    name { "Example SMTP Endpoint" }
+    hostname { "example.com" }
+    ssl_mode { "None" }
+    port { 25 }
+  end
+end