Some more tests for CheckoutSelectionView. Test for OrderConfirmView, and a bit of cleanup.

I added a test case for the OrderConfirmView, which had no tests, and two addition test cases for the CheckoutSelectionView. One tests that the order info is saved on the session if there is no user. The other tests the save_extra_info_to_order method.

While I was there I renamed ShippingBillingViewTestCase to CheckoutSelectionViewTestCase - the name of the view being tested. Also got two typos in comments in shop/views/checkout.py

shop/tests/views_checkout.py still needs a bit of reorganization, which I would like to get around to later.