DEV: Stablize the multisite tests.

SiteSettingExtension triggers message bus which re-establishes a
DB connection in `SiteSettingExtension#process_message`. That happens
concurrently and a test that requires a connection to the db will
fail when the reconnection is happening.

spec/support/site_settings_helpers.rb

@@ -0,0 +1,11 @@
+module SiteSettingsHelpers
+  def new_settings(provider)
+    Class.new do
+      extend SiteSettingExtension
+      # we want to avoid leaking a big pile of MessageBus subscriptions here (1 per class)
+      # so we set listen_for_changes to false
+      self.listen_for_changes = false
+      self.provider = provider
+    end
+  end
+end