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/app/services/unqueue_message_service_spec.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe UnqueueMessageService do
+  let(:server) { create(:server) }
+  let(:logger) { TestLogger.new }
+  let(:queued_message) { create(:queued_message, server: server) }
+  subject(:service) { described_class.new(queued_message: queued_message, logger: logger) }
+
+  describe "#call" do
+    context "when the backend message does not exist" do
+      it "deletes the queued message" do
+        service.call
+        expect(logger).to have_logged(/unqueue because backend message has been removed/)
+        expect { queued_message.reload }.to raise_error(ActiveRecord::RecordNotFound)
+      end
+    end
+
+    context "when the message is not ready for processing" do
+      let(:message) { MessageFactory.outgoing(server) }
+      let(:queued_message) { create(:queued_message, :retry_in_future, message: message) }
+
+      it "does not do anything" do
+        service.call
+        expect(logger).to have_logged(/skipping because message isn't ready for processing/)
+      end
+    end
+
+    context "when there are other messages to batch with this one" do
+      context "when the backend message of a sub-message has been removed" do
+        it "removes the queued message for that message"
+      end
+    end
+  end
+end