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/helpers/message_db_mocking.rb

@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+module GlobalMessageDB
+
+  class << self
+
+    def find_or_create
+      return @db if @db
+
+      @db = Postal::MessageDB::Database.new(1, 1, database_name: "postal-test-message-db")
+      @db.provisioner.provision
+    end
+
+    def exists?
+      !@db.nil?
+    end
+
+  end
+
+end
+
+RSpec.configure do |config|
+  config.before(:example) do
+    @mocked_message_dbs = []
+    allow_any_instance_of(Server).to receive(:message_db).and_wrap_original do |m|
+      GlobalMessageDB.find_or_create
+
+      message_db = m.call
+      @mocked_message_dbs << message_db
+      allow(message_db).to receive(:database_name).and_return("postal-test-message-db")
+      message_db
+    end
+  end
+
+  config.after(:example) do
+    if GlobalMessageDB.exists? && @mocked_message_dbs.present?
+      GlobalMessageDB.find_or_create.provisioner.clean
+      @mocked_message_dbs = []
+    end
+  end
+end