Add method to stub current_user in component tests

We're choosing `sign_in` instead of `login_as` because IMHO component
tests are more similar to controller tests than they are to system
tests.

spec/components/machine_learning/comments_summary_component_spec.rb

@@ -7,7 +7,7 @@ describe MachineLearning::CommentsSummaryComponent do
   before do
     Setting["feature.machine_learning"] = true
     Setting["machine_learning.comments_summary"] = true
-    allow(controller).to receive(:current_user).and_return(nil)
+    sign_in(nil)
   end
 
   it "is displayed when the setting is enabled" do