DEV: Fix flaky specs (#13226)

Some specs failed when `LOAD_PLUGINS=1` was set while migrating the test DB and the narrative-bot plugin disabled the `send_welcome_message` site setting.

spec/models/user_spec.rb

@@ -153,6 +153,8 @@ describe User do
     let(:user) { Fabricate(:user) }
 
     it 'enqueues the system message' do
+      SiteSetting.send_welcome_message = true
+
       expect_enqueued_with(job: :send_system_message, args: { user_id: user.id, message_type: 'welcome_user' }) do
         user.enqueue_welcome_message('welcome_user')
       end