Add expectations in tests confirming account

One of these tests has failed once because there wasn't a user with the
right confirmation token. While I haven't been able to reproduce the
issue, there's a chance it's caused by a `visit` call to the
confirmation path which might start before the redirect request to the
successful sign up page has finished.

I'm not sure this is the case, though, but, worst case scenario, if the
test fails again we'll know it isn't because of a missing expectation.

spec/system/users_auth_spec.rb

@@ -392,6 +392,9 @@ describe "Users" do
 
         fill_in "Username", with: "manuela2"
         click_button "Register"
+
+        expect(page).to have_content "To continue, please click on the confirmation link that we have sent you via email"
+
         confirm_email
 
         expect(page).to have_content "Your account has been confirmed"