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/webhook_delivery_service_spec.rb

@@ -3,7 +3,7 @@
 require "rails_helper"
 
 RSpec.describe WebhookDeliveryService do
-  let(:server) { GLOBAL_SERVER }
+  let(:server) { create(:server) }
   let(:webhook) { create(:webhook, server: server) }
   let(:webhook_request) { create(:webhook_request, :locked, webhook: webhook) }
 
@@ -16,10 +16,6 @@ RSpec.describe WebhookDeliveryService do
     stub_request(:post, webhook.url).to_return(status: response_status, body: response_body)
   end
 
-  after do
-    server.message_db.provisioner.clean
-  end
-
   describe "#call" do
     it "sends a request to the webhook's url" do
       service.call