DEV: Isolate multisite specs (#13634)

Mixing multisite and standard specs can lead to issues (e.g. when using `fab!`)
Disabled the (upcoming https://github.com/discourse/rubocop-discourse/pull/11) rubocop rule for two files that have thoroughly tangled both types of specs.

spec/components/site_setting_extension_multisite_spec.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+describe SiteSettingExtension, type: :multisite do
+  before do
+    MessageBus.off
+  end
+
+  after do
+    MessageBus.on
+  end
+
+  let(:provider_local) do
+    SiteSettings::LocalProcessProvider.new
+  end
+
+  let(:settings) do
+    new_settings(provider_local)
+  end
+
+  it "has no db cross talk" do
+    settings.setting(:hello, 1)
+    settings.hello = 100
+
+    test_multisite_connection("second") do
+      expect(settings.hello).to eq(1)
+    end
+  end
+end