Define and apply matcher :be_rendered

Add this changes to make components specs clear

spec/components/shared/banner_component_spec.rb

@@ -82,13 +82,13 @@ describe Shared::BannerComponent do
 
       render_inline Shared::BannerComponent.new("debates")
 
-      expect(page.native.inner_html).to be_empty
+      expect(page).not_to be_rendered
     end
   end
 
   it "does not render anything given nil" do
     render_inline Shared::BannerComponent.new(nil)
 
-    expect(page.native.inner_html).to be_empty
+    expect(page).not_to be_rendered
   end
 end