Implement pending email digest test

spec/system/emails_spec.rb

@@ -304,6 +304,7 @@ describe "Emails" do
       notification2.reload
       expect(notification1.emailed_at).to be
       expect(notification2.emailed_at).to be
+      expect(email_digest.notifications).to be_empty
     end
 
     scenario "notifications moderated are not sent" do
@@ -320,8 +321,6 @@ describe "Emails" do
 
       expect { open_last_email }.to raise_error "No email has been sent!"
     end
-
-    xscenario "Delete all Notifications included in the digest after email sent"
   end
 
   context "User invites" do