DEV: Resolve stat registration flaky tests (#29084)

spec/requests/site_controller_spec.rb

@@ -46,6 +46,8 @@ RSpec.describe SiteController do
   end
 
   describe "#statistics" do
+    after { DiscoursePluginRegistry.reset! }
+
     it "is visible for sites requiring login" do
       SiteSetting.login_required = true
       SiteSetting.share_anonymized_statistics = true

spec/serializers/about_serializer_spec.rb

@@ -44,6 +44,8 @@ RSpec.describe AboutSerializer do
   end
 
   describe "#stats" do
+    after { DiscoursePluginRegistry.reset! }
+
     let(:plugin) { Plugin::Instance.new }
 
     it "serialize exposable stats only" do