Make sign in spec more stable

This spec is very flaky. Let's use some more page content expectations to give Capybara time to find the elements

Author: tvdeyen

spec/features/spree/sign_in_spec.rb

@@ -245,8 +245,11 @@
       # Logout
       click_link 'Logout'
 
+      expect(page).to have_content "Signed out successfully"
+
       # Step 2: As a guest, add a product to the cart with quantity 2
       visit spree.product_path(@product)
+      expect(page).to have_field "quantity"
       fill_in 'quantity', with: 2
       find('#add-to-cart-button').click