Make component tests fail when using within

This way we avoid writing useless tests which always pass.

spec/rails_helper.rb

@@ -18,6 +18,10 @@ module ViewComponent
     def sign_in(user)
       allow(controller).to receive(:current_user).and_return(user)
     end
+
+    def within(...)
+      raise "`within` doesn't work in component tests. Use `page.find` instead."
+    end
   end
 end